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UPDATED Apr. 27, 2022, at 1:00 PM

All The Science You Need To Make Your COVID-19 Decisions

We’re tracking the research so you don’t have to.

If there's one thing we've learned since March 2020, it's that pandemics are all about hard decisions. It's hard to keep track of the information that helps us make those choices — let alone notice or remember when new science and expert recommendations come along. At FiveThirtyEight, we want to help. We’ve read the science and have come up with broad assumptions you can make based on where the evidence is. When the science changes, so will the assumptions: We’ll be updating this page regularly as new research is published.

We think these assumptions will help you more easily make decisions for yourself and your family. (But do let us know if there are risk-assessment questions you think we’re leaving out.) We want this tool to be something that helps take the stress out of decision-making so that you can worry more about the best way to live and less about the virus.

What We're Tracking:

If there's one thing we've learned since March 2020, it's that pandemics are all about hard decisions. It's hard to keep track of the information that helps us make those choices — let alone notice or remember when new science and expert recommendations come along. At FiveThirtyEight, we want to help. We’ve read the science and have come up with broad assumptions you can make based on where the evidence is. When the science changes, so will the assumptions: We’ll be updating this page regularly as new research is published.

We think these assumptions will help you more easily make decisions for yourself and your family. (But do let us know if there are risk-assessment questions you think we’re leaving out.) We want this tool to be something that helps take the stress out of decision-making so that you can worry more about the best way to live and less about the virus.

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What test should I take if I’ve been exposed to COVID-19, and when?

What you can assume

Instant at-home tests and PCR tests can both be useful and continue to be effective at identifying cases of omicron-variant illness. PCR tests are becoming harder to get, especially if you are uninsured, though you can get eight free instant tests through the mail. But it matters when you're using them and what you're trying to do, and omicron should change some of your testing procedures.

Why we make this assumption

  • The instant at-home tests are rated as less sensitive overall.
    Sensitivity is defined as the likelihood that a test will spot a potential infection. High-sensitivity tests are unlikely to produce a false negative — that is, send you out into the world thinking you’re healthy when you actually have COVID. The tests with the highest sensitivity are molecular (PCR) tests — the sort offered at drive-through clinics and state-run testing centers since early in the pandemic. These rely on specialized laboratory equipment that can find even tiny scraps of viral RNA and then make enough copies of the virus's genetic material so that its presence is detectable. The tests can take several forms, including the ol’ swab up the nose, a saliva test or a throat swab. If you’re getting the nasal-swab version, the highest sensitivity comes from the tests in which trained health care providers put the swab way up in the back of your nasal cavity. Less intrusive ones work but are more likely to produce a false negative. The at-home kits are antigen tests, which are different (and less sensitive) than molecular tests. The antigen tests are designed to spot relatively large pieces of viral protein — something you're more likely to have if you have a high viral load. Compared with molecular tests, these at-home tests will turn up a false negative more often, especially in asymptomatic people. For example, in a paper that hasn't yet been peer-reviewed, researchers compared PCR and antigen test results at a walk-in testing center in San Francisco. They found that the sensitivity of an antigen test varied a lot, depending on how much virus was present in the person's system; it ranged from 95 percent sensitivity for those with a high viral load, to 65 percent in those with a low viral load. Among the 61 people in the study who had very low viral loads but still tested positive on PCR, none tested positive on an antigen test.
  • The instant at-home tests are about as sensitive as PCR tests for the time period when an infected person is most likely to be contagious.
    Compared with at-home antigen tests, PCR tests are less likely to deliver a false negative over a longer period of time, but when it comes to the critical three days when a COVID-infected person is most contagious, the at-home antigen tests and the PCR tests are both very accurate, according to Michael Mina, a professor of epidemiology at Harvard University. (The U.K. has used antigen tests — which they call "lateral flow tests" — to great effect in stemming the spread of COVID, and a chart in this story can show you the relationship between infectiousness and the effectiveness of both kinds of tests.) Before and after the peak of infectiousness, the PCR tests are likelier to spot a COVID infection. But at-home tests can be used effectively as a tool for public health, too. One example Mina uses is if everyone had to take a test at the beginning of a school day. If there were five people actually infected, the PCR tests would be more likely to identify them all — but you'd need to wait for the results, and in the meantime, all those people would be wandering around the school for a day or two. The instant antigen tests, meanwhile, might only identify four of the infected people, but you'd know immediately and could send them home. And the other one might be identified by a rapid test the next morning, if you were testing every day. Overall, fewer people are exposed and less disease is spread.
  • If you choose at-home tests, you should test more than once.
    Because the sensitivity of the at-home antigen test changes over time, you'll need to test more than once, which is why most kits are sold with two tests in each pack and tell you to test on consecutive days. The tests are most useful if people are doing them regularly over long periods of time — like, say, if school kids or workers were testing daily as part of routine outbreak prevention. The more likely it is that you've been exposed but are getting a negative instant result, the more you should want to confirm that result. If you're symptomatic and testing negative, you should self-isolate and test for the next two consecutive days. Overall, choosing a test is a matter of balancing accuracy, speed of results and frequency of testing. And there is still a lot we have to learn about the omicron variant that could change testing strategy in the future.
  • The research on whether antigen tests remain as sensitive to omicron as they were to earlier strains is mixed. It's worth continuing to use them, but you should assume a possibility that false negatives are more likely.
    Early in the omicron wave, the FDA reported that the most common commercially available antigen tests worked well in laboratory studies and were able to detect the new variant reliably. That is no longer what the agency's site says. Studies done with live virus — as opposed to earlier ones done with heat-inactivated virus — showed that antigen tests may have reduced sensitivity to omicron. That means there might be a higher likelihood of false negatives. Since that announcement was published on the FDA site, other research — both in the laboratory and the real world — has turned up mixed results. Most of this research hasn't yet been peer-reviewed. Some studies, including one done in a lab using live virus, show antigen tests retained their accuracy with omicron. Other studies, including a peer-reviewed laboratory study published in February, found that the tests needed higher amounts of omicron virus to produce a positive result compared with delta. None of this means antigen tests are useless. It just means that you should assume they might produce a false negative in the first couple days of an infection, and that you should test repeatedly over several days to get a trustworthy result. This is also why Michael Mina, a Harvard professor of epidemiology who has been arguing for increased use of rapid tests, says that if you are testing before attending an indoor event, you need to be testing immediately before the event. A faster-spreading virus means a virus that is multiplying faster in your body, so your results one or two days before the event might not give you the most accurate understanding of infection status. For best accuracy, test a couple days prior and then test again within a few hours of the event. The Ontario COVID-19 Science Advisory Table has a nice brief that breaks this all down very well.
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How long do I have to quarantine if I’ve been in contact with someone who has COVID-19? And what do I do if I am sick?

What you can assume

The CDC shortened the length of its recommended quarantine on Dec. 27 from 10 days past the point of exposure to five if you are not vaccinated or if you received your primary vaccination a while ago (i.e., more than six months ago for an mRNA vaccine, or more than two months ago for Johnson & Johnson) and have not received your booster. Anyone who has received their booster, whose primary vaccination happened within two or six months (depending on vaccine type) or who has had a documented case of COVID-19 in the last 90 days doesn't need to quarantine after being exposed to an infected person. The agency shortened its recommended self-isolation period for those who test positive as well. They also have a new app on their website to calculate the timing of quarantine, isolation and testing for you, based on a series of questions.

Why we make this assumption

  • Recommended isolation after a positive test is shorter now, but that doesn't mean we know that you, personally, are infectious for a shorter period of time.
    Quarantine means staying home and away from other people after recently being in close contact with someone diagnosed with COVID while you wait to see whether you’ve been infected. Isolation means staying home and away from other people after you have tested positive for COVID. The CDC now says that everyone, regardless of vaccination status, should isolate for at least five days if they test positive. If at the end of that period you have no symptoms, the CDC says that you can end isolation but that you should continue to wear a mask around others for another five days. This is different than just masking up to go to the grocery store. You should be wearing a mask more extensively during this time than you do when you're sure you're healthy, even if you are vaccinated and boosted. Here's why: Some data suggests people can clear an infection faster than previously assumed, and that the vaccinated return to health faster than the unvaccinated. A study analyzing infections of NBA players, staff and other league affiliates, for example, found vaccinated participants were clearing an infection in 5.5 days on average, and unvaccinated participants in 7.5 days. But there are limitations to this. Omicron-specific data suggests that the variant might result in people being contagious for longer. All studies on this are narrow and may not be something you can extrapolate to everyone, and in any case, they're talking about averages. Your results may not be the same as an average. You should assume that you can still be infectious beyond five days, even if you are vaccinated. This discrepancy between what is average and what is possible is why other countries and the World Health Organization have different (and often stricter) recommendations than the CDC. The WHO, for example, recommends a minimum of 7 days of quarantine, even for people who are up to date on all their shots. Although the shortened CDC isolation and quarantine guidelines initially made no mention of testing as part of the strategy, it now emphasizes it for quarantine, though it's still not part of how the CDC determines when someone can leave isolation. This is partly because we know that PCR tests can produce a positive result long after a person has stopped being infectious. However, rapid antigen tests — the at-home kind from the drugstore — are good at telling you whether you are infectious. (See our section on testing for more information.) If you have COVID, you should use them as a better determinant of when it is safe for you, personally, to leave quarantine and isolation without risk of spreading COVID to friends, family, coworkers and the general public.
  • Testing and masking are more critical under the new quarantine guidelines.
    Quarantine means staying home and away from other people after recently being in close contact with someone diagnosed with COVID while you wait to see whether you’ve been infected. Isolation means staying home and away from other people after you have tested positive for COVID. The CDC has shortened its quarantine guidelines significantly. People who have received a booster, whose primary mRNA vaccines were less than six months ago or whose primary Johnson & Johnson vaccine was less than two months ago do not have to quarantine — but they should mask for 10 days and be prepared to test and stay home if any symptoms develop. People who are unvaccinated or whose primary vaccinations are outside that range and who haven't gotten a booster should stay home for five days and mask for another five. The CDC also recommends testing at the end of the five days of quarantine, to make sure you aren't infected. If you test positive, you should then enter isolation — and start counting days all over again. This difference in length of quarantine for these two groups is based on data that suggests people who have received their booster shot are significantly less likely to contract the omicron variant than those who have not received a booster or who are unvaccinated. There's also evidence that infectiousness peaks within the first week of a case of COVID. That said, however, there does not seem to be solid omicron-specific data for length of infectiousness. Some preliminary data from Japan suggests it can last longer than for previous variants, and we do know that omicron can evade booster immunity. If you choose to follow these new CDC guidelines and shorten your quarantine, you should be thinking of your mask as important to preventing spread — it should fit better and prevent more particles from escaping than most cloth masks are capable of. Paying attention to symptoms and testing are also a key part of preventing transmission effectively under these guidelines. Michael Mina, an epidemiologist who has extensive experience studying rapid antigen tests, says that for breakthrough infections of omicron in vaccinated people, symptoms seem to be starting before those people become infectious. If you have symptoms, take a test. If the test is negative and you still have symptoms, test on consecutive days because the outcome of a test that is geared to identifying when you are infectious could change.
  • The new, shorter quarantine and isolation guidelines have a scientific basis — but they also seem to be based on economic and political demands.
    Earlier in the fall, Michael Springer, a professor of systems biology at Harvard University, told us that most infected people will show symptoms or a positive test result within 14 days of exposure. But there’s a reason quarantine periods aren’t 14 days long, he said. Recommended quarantine timelines are based both on scientific evidence about how long it takes for COVID to make a person sick or contagious, and on attempts to balance that with other concerns, like economic constraints, child care availability and plain ol' not wanting to be trapped in the house that long. That's why the WHO includes several different levels of recommendations, depending on economic and public health pressures in the specific country. In December, the CDC received letters from several airlines, including Delta, asking that these isolation times be shortened for breakthrough cases in vaccinated people. The subsequent decision to do so seems to be in response to these letters. Likewise, former CDC Director Tom Frieden described the new guidelines as “taking the world as it is not as we wish it would be.” This is not necessarily a bad thing, but experts were very critical of the fact that rapid antigen tests were not initially being heavily recommended or required as a part of the CDC’s decision. However, there was also a nationwide shortage of those tests at the time. Now, however, they're easy to find. But while every household can get eight tests free from the government, and some Americans can use HSA and FSA funds to purchase tests, the tests remain expensive. It is important to note that the new CDC guidelines are one way to interpret and make use of data while balancing other factors — but they are not the only way. In countries where rapid tests are not in short supply and are cheap, health care agencies recommend very different quarantine and isolation rules.
  • If you test positive — and are at high risk of severe illness — there are medications you can take that have been shown to improve outcomes. But you will have to move quickly.
    While social media has been full of "treatments" for COVID that don't work (like ivermectin), there are some real medications that do. The bad news is that they’re often difficult to get. Monoclonal antibodies — basically just manmade versions of the antibodies your body produces to fight off infection — as well as two kinds of antiviral medications have been approved. Unfortunately, to get the benefits of these medications, you have to start taking them very quickly after diagnosis. A course of antivirals must begin within five days of developing symptoms. In addition, only some people are eligible to take these medications. The government has tried to set up a national system of centers where people can go to be diagnosed and get a prescription for the antiviral drugs at the same time. It's called Test To Treat, and you can use a map to try and find a location near you. However, the system has been heavily criticized (as of April 2022) for not featuring on its official map all the locations that can test and treat, for including places that do not reliably have access to the drugs or charge hefty fees for prescribing them and for leaving large swaths of the country without any nearby centers. Meanwhile, some pharmacies have ended up with too many antivirals while the drugs remain elusive for patients in other places. Test To Treat is a good place to start, but you should understand that it may not make getting the drugs super simple. One other option to consider is fluvoxamine, an antidepressant that has been shown in multiple studies to reduce the risks of hospitalization and serious side effects. This drug does not yet have approval for COVID treatment, but infectious disease experts have begun suggesting it could be used off-label. You'd have to consult your doctor.
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Do masks protect you and others from COVID-19?

What you can assume

They do. There's also a growing body of evidence suggesting that mandates — which take a community-wide approach to reducing spread of disease in the community — effectively reduce transmission, hospitalizations and death in communities where they are implemented.

Why we make this assumption

  • Masking reduces the spread of COVID, especially in places where more people are masking, which is why mandates within schools, cities, counties and states matter.
    Back in March and April 2020, there was genuine debate among scientists about whether masking was significantly effective at preventing the spread of COVID, especially compared with interventions like social distancing. That has not been the case for well over a year. In January 2021, a systematic review of various types of studies concluded that mask-wearing — especially if widely adopted — was effective at reducing COVID transmission. An September 2021 study of nearly 350,000 adults across 600 villages in Bangladesh found that widespread use of surgical masks, even at levels below 50 percent, reduced the likelihood of contracting COVID by 11 percent — and almost 35 percent for people over 60. And research in the U.S. has shown that masking during surges can help curtail spread and protect populations. Further studies have shown that mandates were effective at reducing COVID in the places they were implemented. For example, when mask mandates were first implemented in some (but not all) Kansas counties in 2020, those counties had COVID rates three times higher than the counties that did not, but within months, that trend had reversed, with the non-mandate counties having rates more than two times higher than those with mandates. Another study found that even in states without mask mandates, county mandates were associated with a nearly 17 percent reduction in COVID cases over the 30 days after implementation. There is also evidence for the effectiveness of mask mandates in schools. A March 2022 study in the journal Pediatrics compared 61 districts across nine states during the fall semester of 2021. The study found that most people were catching COVID outside of school, rather than in it, but that mask mandates in the schools still reduced transmission. For every 100 community-acquired cases, universally masked districts had 7.3 cases of school-transmitted COVID and optionally masked districts had 26.4.
  • CDC masking guidelines no longer reflect local transmission rates — they are focused more on hospitalizations.
    If you are deciding whether to wear a mask indoors based on recommendations made by the CDC, know that the agency's metrics for determining which counties should have universal masking have changed. Risk levels were previously determined by the number of cases in a community, but beginning February 24, 2022, the agency switched to a system that considers a combination of three factors: new COVID-related hospital admissions from the previous week; the percentage of hospital beds currently occupied by COVID patients, and new COVID cases per 100,000 people over the previous week. This change is largely based on the fact that so many people have now either been vaccinated or acquired immunity through infection. Neither vaccine nor infection-mediated immunity is a perfect barrier against getting infected again, but both reduce the risks of severe illness. These guidelines drastically changed the number of counties that were considered high risk and in need of universal masking. Previously, 95 percent of US counties were considered high-risk and had universal masking recommendations. Under the new metrics, that number fell immediately to 37 percent of counties. The new mask guidelines were also extended to schools, with the CDC only recommending universal masking in K-12 schools in those counties with high community-risk levels. This change has been controversial among scientists — some support it, some don't — though not because of the idea that masking may be necessary at some times and unnecessary during others. Instead, the debate largely stems from a question of which metrics should be used to determine low risk and whether the metrics chosen by the CDC truly do enough to protect the most vulnerable, such as immunocompromised people and young children. There's also concern about the way the new guidelines don't seem to factor in the risks posed by long COVID. Earlier in February, for example, a group of public health researchers published an open letter requesting that mask guidance take vaccination rates among children into account.
  • Masks still work, even if you're the only one using them.
    With the elimination of mask mandates from most schools, public buildings and even planes and public transportation, you can no longer rely on community masking to reduce risk in a large, systemic way. The good news is that one-way masking is still useful. Even if you're the only person masked up at the gym or your office or on a flight, you are still protecting yourself more than you would be without a mask. A study from before the pandemic found that average people using an N95 mask can block their exposure to about 80 percent of airborne particles. If you are someone with a high risk of serious COVID side effects, you might want to consider adding a face shield on top of a mask.
  • There's no evidence that mask-wearing is broadly harming children's development.
    Some are worried that very young children may experience developmental delays because the adults around them are wearing masks, potentially preventing them from picking up on facial recognition, emotional cues and speech acquisition. This is not an unreasonable concern to have — researchers have published several papers that express similar concerns — but there isn't much to suggest this is actually happening. That's partly because research has been limited, but also the studies that have been done have found no evidence to support a clear, consistent detrimental impact caused by masking. For example, a study of 7- to 13-year-olds found that kids could successfully infer emotion from masked faces — at about the same rate as they could from faces wearing sunglasses. And a study of interactions between infants and mothers found that while infants reacted to mothers taking masks off and putting them on, wearing a mask did not seem to damage the connection between the two or cause infants distress. Wearing a mask was not analogous to the problems caused when caregivers are emotionally unresponsive or inexpressive. This is not to say, however, that young children aren't experiencing various kinds of delays — just that masking, specifically, does not currently appear to be the cause. Nature published a story in January that got more deeply into evidence of developmental delays and what researchers think is causing them. One study that hasn't yet been peer-reviewed found that babies born during the pandemic were scoring almost two standard deviations lower on a battery of cognitive tests than babies born earlier — and that gross motor skills seemed to be the area most affected. Researchers told Nature that they thought a big factor here was the fact that these children had less access to peers, physical play and interactions with other adults — something caregivers have told scientists they worry about, as well. This theory is backed up by the finding that young children who were in child care or preschool during the pandemic in the U.K. — where face masks have been required in public places until very recently — displayed fewer deficits than those who were not. However, there are some subpopulations (both adults and children) for whom masking does pose speech and communication problems. Numerous studies have found that people who are hard of hearing have significantly more trouble understanding speech and communicating with others because of the way that masking dampens sound and obscures the mouth, and that this can lead to intense feelings of isolation and stress. Some studies found that clear masks can help with this — others found clear masks didn't make a difference.
  • Your cloth mask is not as effective as you’d like it to be.
    Research has shown that not all types of masks are equal. For example, experts have been warning for a while that bandanas and thin fabric neck gaiters aren’t effective at blocking transmission of airborne particles. But additional evidence now shows that even multilayered cloth masks aren’t as effective as well-fitting surgical or N95 masks. One study that hasn’t yet been peer-reviewed found that fabric face masks (which have a lot of variability in design and material) blocked between 62.6 to 87.1 percent of fine particles, while surgical masks blocked an average of 78.2 percent and N95s blocked 99.6 percent. Some countries, including France, have banned DIY cloth masks and stipulated that only certain types of factory-produced varieties are acceptable. Early in the pandemic, cloth masks were promoted as a way to get people masked up during a shortage of higher-quality masks, but mask supply has since caught up with the demand in the United States. N95s — and the Chinese- and Korean-made equivalent KN95s and KF94s — are widely available now, and, as of Sept. 10, the CDC says they should not just be reserved for medical staff. Proper fit is a big deal here, though. A poorly fitting surgical mask is not an upgrade from a well-fitting cloth mask.
  • You can get multiple reuses out of N95 and KN95 masks.
    Remember that these masks were, prior to COVID-19, industrial tools with regulations and recommendations geared toward those uses. They were intended to be thrown away after a single medical procedure or factory shift, so the idea got down to the public that we should also throw these masks away after a few hours or a day of use. But that is not true. In fact, health care workers have reused these masks successfully in the past. The main issue is whether the mask will still fit to your face effectively after multiple uses. There's not a ton of evidence on the fine details of mask reuse, but studies have shown that you can get at least two or three reuses with little change to fit. Outside formal scientific papers, people like Aaron Collins, an aerosol engineer, have been conducting tests of masks for the way people use them now. He's found that, depending on the brand, these masks can maintain high levels of filtration quality through 30-40 hours of light, everyday use.
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How likely am I to contract and spread COVID-19 if I’m vaccinated?

What you can assume

Breakthrough cases have become common since omicron became the dominant variant. The likelihood of an individual contracting COVID-19 depends on a combination of primary vaccination status, booster status, masking, other preventative behavior and how common cases are in your community. Vaccinated people can spread the omicron variant more easily than past variants, and studies suggest you're probably more infectious before symptoms kick in than after.

Why we make this assumption

  • Vaccinated people are still less likely to catch COVID, especially if they have been boosted.
    You can’t spread the virus if you don’t contract it. And the good news is that vaccination does still protect you against infection. Data from the U.K. found that vaccine effectiveness against omicron fell even further, depending on how long it had been since the person was vaccinated with the Pfizer vaccine. Effectiveness fell to less than 9 percent in people who had been vaccinated 25 weeks ago or longer. That study showed that a booster shot could bring effectiveness back to about 67 percent, though it did decline again to about 46 percent by 10 weeks post-booster. Some people might be more likely to have a breakthrough infection than others. In a recent study that has not yet been peer-reviewed, for example, pregnancy almost doubled the likelihood of a breakthrough infection.
  • It is much easier for vaccinated people to spread omicron than previous variants.
    A U.K. study that has not yet been peer-reviewed found that a booster does reduce the likelihood of spreading omicron — because it prevented people from catching the virus — but boosted people who did get omicron were more likely to spread it than boosted people who got delta. Omicron has definitely changed the calculus here, probably by making it just more likely for vaccinated people to get infected at all.
  • There are specific complications that put people at higher risk of severe side effects if you spread COVID9 to them — and those complications are common.
    Vaccinated people can spread COVID to both vaccinated and unvaccinated people, so it is worth considering the risks others might face if you spread the virus to them. (We address long COVID and other long-term complications in another part of this dashboard.) These higher-risk groups are a combination of two things: direct risks (you have a physical condition that makes severe COVID more likely) and statistical risks (people who have been statistically more likely to die, which is probably less about their group identity and more about how society treats that group and the access they have to health care). The omicron variant produces less severe health outcomes, but because it is also easier to transmit and more people have acquired it, it's also associated with increases in hospitalizations. Being unvaccinated remains a big predictor of severe outcomes, even during and after the omicron wave. For example, during the omicron wave, the hospitalization rate among vaccinated adults peaked at about 23 per 100,000. That same week, the hospitalization rate among the unvaccinated was 150 per 100,000. Age remains the biggest risk factor, with more than 81 percent of COVID deaths happening in people over the age of 65. There is also a long list of conditions that have strong evidence suggesting they increase the risk of severe COVID outcomes. These include cancer, diabetes, pregnancy, chronic kidney disease and a history of smoking. There are also still racial disparities in severity of COVID illness, even after disparities in vaccination rates have narrowed (and in the case of Hispanic Americans, reversed). Scientists think these statistical risk correlations are likely tied to factors like health care access and social stigma, which have led to minority communities experiencing increased rates of some of the chronic diseases that put people at higher risk for complications after contracting COVID. The important takeaway here is that it's not rare for people to be over the age of 65 or to have one or more of the conditions that can increase risk. Age and medical risk factors suggest that nearly 40 percent of Americans fall into a higher-risk group. If you want to understand your personal risk better — or make decisions about safety precautions in light of the risk levels of people you'll be spending close time with — there is a National Science Foundation-funded relative-risk calculator app that can help.
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Who benefits from a booster shot, and when should we get one?

What you can assume

The Centers for Disease Control and Prevention now says everyone over the age of 12 is eligible to get a booster shot. Given what we now know about the waning efficacy of the original series of shots in the face of the omicron variant, everyone in this category benefits from a booster. In addition, people over 50, anyone over 12 who is severely immunocompromised and people who have gotten only a non-mRNA vaccine are eligible for a second booster if their first was at least four months ago. .

Why we make this assumption

  • Vaccines are less effective at preventing infection and transmission than they once were.
    A U.S. study that compared data from different time periods during the pandemic found that the effectiveness of two does of the mRNA vaccine fell to about 65 percent against omicron, though a booster shot could raise it to 86 percent. Data from the U.K., meanwhile, found that vaccine effectiveness against omicron fell even further, depending on how long it had been since the person was vaccinated with the Pfizer vaccine, with effectiveness falling to less than 9 percent in people who had been vaccinated 25 weeks or longer. That study showed a booster shot could bring effectiveness back to about 67 percent, though it did decline again to about 46 percent by 10 weeks post-booster. The vaccines, and particularly the boosters, still make you less likely to be hospitalized. The studies tracking vaccine effectiveness continue to find evidence that vaccination improves individual outcomes and results in less severe illness if you do catch it. We talk more about data on infection prevention and hospitalization in our section on spreading COVID.
  • Older people get a much bigger benefit from boosters than younger people do.
    That's not to say, though, that the booster has no benefit for younger people. In evidence presented to the CDC by Pfizer in November, fully vaccinated people between the ages of 16 and 55 who got a booster decreased their risk of contracting COVID over the next three months by 97 percent, compared with a group who received a placebo.
  • There still seems to be a lot of uncertainty among scientists about how many boosters make sense, how frequently and for whom.
    The frustration in the scientific community around this issue was clear in an April 2022 meeting of the CDC's advisory board of independent vaccine experts. Both FDA and CDC officials have said in the past that boosting everybody every four or five months is not a good long-term strategy, just on the basis of logistics and public buy-in alone. But the panel of scientists complained that there's not a good plan in place for an alternative — like authorizing new variant-specific vaccines — or even for deciding in an evidence-based way how often people will need new boosters of existing vaccines. Scientists have told us, and other journalists, that there no evidence to suggest that boosters are doing anything dangerous to us, like weakening our immune systems or producing new strains in people who get them every four months … but there's also just not much evidence about repeat boosting at all, because that kind of regimen has never been done before.
  • Infection with COVID can provide similar levels of immunity as vaccination, just with more risks of side-effects. A combination of vaccination and infection might provide the highest level of immunity.
    Research over the last two years has shown that COVID infections can confer immunity that can be as effective as the immunity conferred by vaccines. Growing evidence continues to show that a combination of infection and vaccination is probably the most effective at preventing future infections (though there is no omicron-specific information on this yet). But it's not really clear whether that means infections from omicron will end the pandemic by making enough people immune to future waves. That could happen, but what we do know about natural immunity suggests the future may not be so straightforward. For instance, mild and asymptomatic COVID infections have long been associated with less effective immunity later. Another potential problem: Omicron might be different enough from other variants that immunity to it might not mean much when new variants come along.
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What do we know about the rate of long COVID?

What you can assume

Long COVID is real, and it is possible for anyone to get it. That includes kids and vaccinated people — although both are probably less likely to end up with long COVID than unvaccinated adults. Scientists are still figuring out how common long COVID is, but an April 2022 meta-analysis and systematic review of the research that had been peer-reviewed as of July of 2021 found that global average rates could be as high as 43 percent. In addition, there is increased understanding of long-term complications of COVID-19 infection beyond long COVID itself.

Why we make this assumption

  • Researchers say most studies of long COVID are still frustratingly vague, flawed and hard to compare with one another.
    It’s still not clear how common long COVID is. Published studies have suggested that long COVID persists at rates anywhere between 4.5 percent and 87 percent. That wild amount of variation has a lot to do with the fact that there is still no universal, set definition of what constitutes long COVID. Self-report surveys have turned up literally hundreds of possible symptoms, and scientists haven’t come to an agreement or been able to conclude how to best classify the label. Some of the most clear benchmarks — like a study that showed one-third of COVID patients who were sick enough to be hospitalized were later rehospitalized for other problems at a higher rate than the general population — don’t necessarily tell us enough to be extrapolated to the broader demographic of mild COVID patients. A lot of the surveys about long COVID also recruit participants from online communities dedicated to long COVID, which therefore might not do a good job of reflecting the phenomenon’s prevalence in the general population. It’s messy.
  • Long COVID is real.
    Just because diagnostic criteria are still confusing and there aren’t solid numbers on prevalence doesn’t mean the syndrome is imaginary. Let’s narrow down the symptom list and focus on fatigue as an example. It’s a symptom that can cause real, life-changing disability and is a known side effect of other viral infections. Studies find that anywhere between 10 percent and 35 percent of former COVID patients are still experiencing significant fatigue that can’t be explained by other illnesses or psychiatric causes six months after their initial infection. Studies have found that long COVID is probably more common in women than in men, in older people than in younger ones and in people who had more symptoms (though not necessarily more severe symptoms) during their initial infection. Another study found some key biological predictors of long COVID: whether the patient had COVID RNA in their blood, which would indicate a higher viral load; whether the patient's blood contained autoantibodies, which attack the body's own tissues are are present in cases of autoimmune disorders, like lupus; whether the person shows signs of reactivation of Epstein-Barr virus, a virus that infects some 90 percent of people in early childhood but usually goes dormant in the body; and whether the patient had Type 2 diabetes before catching COVID. Unfortunately, only diabetes is a predictor you’d likely know about before you get sick with COVID.
  • Preliminary research suggests long COVID is less likely to occur in vaccinated patients and in patients under age 10.
    Vaccination reduces the risk of long COVID by simply reducing the risk of contracting COVID at all, but there is also some recent evidence that breakthrough cases are less likely to develop long-term symptoms. A study of self-reported symptom data from COVID patients in the U.K. found that people who said they were fully vaccinated with two-dose vaccines were about half as likely to still have COVID symptoms 28 days after infection. Likewise, there’s a recent study out of the U.K. that found only 4.4 percent of pediatric COVID patients were still experiencing symptoms after 28 days, and less than 2 percent still had symptoms after 56 days. That study fits with a broader context of research that suggests kids can get long COVID but are less likely to do so than adults. However, again, all of this has to be considered through the reality that research on long COVID is messy and that we will learn more from systematic reviews, meta-analyses and growing bodies of evidence over time than from a single study. Early data from studies that haven't yet been peer-reviewed also suggest that getting vaccinated after infection could reduce the risk and severity of long COVID. A recent peer-reviewed, systematic review of long COVID studies in kids and teens also found some encouraging trends. Having had COVID did seem to increase the risk of some long COVID symptoms — but not by huge amounts. We're talking about an 8 percent increase in the risk of loss of smell and a 5 percent increase in the risk of headaches. For most of the other possible long COVID symptoms, however, having had COVID didn't change the likelihood of those symptoms, which means they were not likely a result of long COVID. Prevalence of long COVID symptoms were higher in older teens than in younger children. And better-designed studies found fewer long COVID symptoms at all.
  • There are long-term side effects of infection besides long COVID.
    This can be hard to parse out because there is not a firm definition of what symptoms do and don't constitute long COVID. But if what you have pictured is the months or years of extreme fatigue and brain fog most frequently described in symptom surveys, you should know that there are still ways COVID can harm you long-term — even if you feel like you fully recovered quickly. A study published in February 2022, for example, found that having had COVID — even a mild, transient case — seriously increased the risk of developing cardiovascular problems later. Having contracted and recovered from COVID increased the risk of stroke in the next year by 52 percent, compared with the control group, while the risk of heart failure increased by 72 percent. Researchers found these risks increased significantly even in people who were under the age of 65 and had no other cardiovascular risk conditions.
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Should I get my child vaccinated?

What you can assume

Yes. The benefits are clear for children 12 and over, and even though effectiveness at preventing transmission dropped dramatically for kids under 12 during the omicron wave, some evidence suggests that the vaccine still reduces severe side effects from infection for this group. The delta and omicron variants are more contagious and more likely to produce breakthrough cases, even though they don't seem to be worse for kids than for adults, either in likelihood of transmission or severity of illness. The raw number of pediatric hospitalizations for COVID-19 went up during the omicron wave, but that's basically because everyone was more likely to get infected. Kids who spend their time around vaccinated adults and teens are at lower risk, but the benefits of vaccinating kids outweigh the risks.

Why we make this assumption

  • The benefits of vaccinating kids outweigh the risks for kids ages 12 and over, even after omicron is taken into account. But this variant seems to have drastically reduced the efficacy of the lower-dose vaccine administered to kids 11 and younger, at least when it comes to preventing infection.
    As of early November, the CDC approved vaccination for children ages 5 and up. That was based on results of Pfizer's clinical trial and an analysis of the potential risks. An independent FDA analysis considered both Pfizer's finding that its vaccine is 91 percent effective at preventing COVID in children ages 5 to 11 as well as the risks of both severe illness and a rare (but mild) vaccine side effect that has caused inflammation of heart tissue, particularly in young men. But Pfizer’s data included no instances of heart inflammation in the children ages 5 to 11 who received the vaccine, despite it occurring in an estimated 71.5 cases per million 16- and 17-year-old boys vaccinated. Ultimately, the FDA concluded that the benefits of vaccination outweighed the risks. Even if the number of kids hospitalized for heart inflammation were higher than the number of kids hospitalized for COVID, getting vaccinated would still be worth it because being hospitalized for heart inflammation is still that much less risky than being hospitalized for COVID. For example, being infected with COVID can cause a severe reaction called multisystem inflammatory syndrome, and studies suggest that vaccination can be more than 90 percent effective at preventing that in kids 12 and up, even if the kid still gets infected. Unfortunately, that data was all gathered before the omicron wave. This variant turned out to be extremely good at evading all kinds of immunity, and preliminary data published (but not yet peer-reviewed) in February 2022 suggests it rendered the lower-dose vaccine given to young children all but useless against infection. Among more than 365,000 children ages 5 to 11, efficacy rates measured between December of 2021 and January of 2022 fell from 65 percent to just 12 percent. It is less clear how much efficacy the vaccines retained against severe illness, hospitalization and death. Those outcomes are rare enough in young children, generally, that it's hard to get a precise understanding of how much vaccines reduce their likelihood. The preliminary data does suggest, however, that the vaccines did retain more effectiveness in this area than they did in preventing infection, broadly. In fact, in April 2022, England's National Health Service approved the use of the Pfizer vaccine in children ages 5 to 11, even though most children in that country are estimated to have already been infected with COVID — largely on the basis of the vaccine's potential to reduce severe side effects and risk of long COVID. But omicron's ability to escape the lower-dose vaccine immunity does mean that kids 11 and under benefit less from vaccination. Since this news came out, Pfizer has published data intended to support authorizing a third dose for kids 5-11.
  • The omicron variant is not more dangerous for kids than previous variants, nor more dangerous than it is for adults.
    This one is a bit confusing because it is true that pediatric hospitalizations for COVID have risen significantly during the omicron wave. But that is not the same as saying that omicron has made COVID more dangerous for children. If lots and lots more people are catching the virus, more are going to end up in the hospital, even if this variant is overall less likely to produce serious illness. It's a numbers game. Death rates in children remain extremely low — with less than 0.02 percent of COVID cases in children resulting in death. The increased numbers of children in the hospital with COVID also reflect the fact that more kids who happen to have COVID are hospitalized for other things. Hospitals have told news outlets that these kinds of cases might make up the majority of children in hospitals with COVID. Meanwhile, data from the U.K. and South Africa suggest that omicron is not more severe in children, and preliminary data from the United States shows the same trendsincluding in children under the age of 5, who cannot yet be vaccinated.
  • Vaccinating those who can be vaccinated protects people who can't.
    We know that vaccines work and that vaccinating kids ages 5 to 17 will help protect kids too young to be vaccinated yet. Data from August shows how severe cases of COVID-19 in children became concentrated in the parts of the country with the lowest rates of adult and teen vaccination. In New England, where more than 72 percent of the eligible population was vaccinated, the daily rate of cases of COVID in people 17 and under was about 13 for every 100,000. In the Southeast, where less than 50 percent of eligible people were vaccinated, the daily childhood COVID rate was nearly 50 per 100,000. Between Aug. 14 and Aug. 27, kids under age 18 were more likely to be admitted to the hospital or ER in regions with lower vaccine coverage than in regions with higher vaccination rates. Vaccination reduces the spread of COVID, thereby helping protect those who can’t be vaccinated yet.
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How can I tell if a new variant surge is happening in my community?

What you can assume

Testing data has gotten less reliable as more people are using at-home rapid COVID-19 tests and fewer cases diagnosed that way are reported to states. At the same time, the CDC has changed their public community-risk map in ways that might not make a local increase in cases clearly apparent to you. However, there are several tools available that can at least give you a better picture of general trends in your area so you can make personal decisions about things like masking or whether it's time to get a booster.

Why we make this assumption

  • The CDC has multiple ways of tracking COVID rates in communities.
    In our section on masking, we talk about the scientific controversy surrounding the CDC's decision to change its basis for masking recommendations from a community-risk map based on the number of cases per country to a new map that combines three metrics: new COVID-related hospital admissions from the previous week, the percentage of hospital beds currently occupied by COVID patients and new COVID cases per 100,000 people over the previous week. This change in methodology had the effect of drastically reducing the number of counties where masks were recommended. But the agency also has other ways that it's tracking the spread of COVID in communities. For example, you can still access maps of community transmission levels, as measured by the agency's old case-based metric, on its Integrated County View page. Just set the data type selection to "Community Transmission". There's also a regular report, created using CDC data and published at the HealthData.gov website, which includes a metric called Rapid Riser Counties. This map shows you which counties meet a series of criteria that suggest COVID transmission is rapidly increasing in that area. Another option is to look at the CDC's National Wastewater Surveillance System maps, which show data and trends from efforts to monitor COVID in sewer water in various counties and cities. All of these maps measure something slightly different, they all have limitations and it is possible for them to show very different pictures of risk for the exact same place. But taken together, they can help you get a better idea of what is happening in your county and whether there's a new variant on the rise where you live.
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How likely am I to catch COVID-19 outside?

What you can assume

It is possible to catch COVID outside, even for the vaccinated, but outdoor activities are much lower-risk than indoor ones. Studies have shown that the risks of transmitting COVID outdoors can be extremely low but are affected by factors like air flow, weather and air pollution. Don't assume that 100 people in a tent is an equivalent risk to 100 people spread out in a park.

Why we make this assumption

  • Studies suggest outdoor interactions are generally much less likely to spread COVID than indoor interactions.
    In November 2020, a systematic review found that less than 10 percent of globally documented outbreaks were associated with outdoor settings. The authors of that study have since clarified that this figure was just an estimate — and a high one at that. It was meant to capture the study’s upper limits and keep people from getting too complacent while outdoors. The authors wrote in June 2021 that the rate of outdoor transmission is probably much less than 10 percent.
  • Outdoor transmission can happen, however, and research suggests it is affected by physical, geographic and meteorological circumstances.
    There have been cases of outdoor transmission, including a frequently cited case where six out of the 92 people who attended an outdoor tent wedding near Houston in early April 2021 contracted COVID. One of those six died. Vaccination was required for attendance at the wedding, but two people who traveled to Houston from India seem to have brought a breakthrough infection with them. Everyone else who got sick had close contact with them. More contagious variants like delta and omicron affect transmissibility even outdoors. Delta, for instance, is more contagious because it produces higher levels of the virus in nasal passages and lungs, so there’s more to spread around when an infected person coughs or sings or talks. And that makes it more likely that someone nearby will get a high-enough dose to catch the disease themselves. But generally speaking, any group activity will be significantly less risky outdoors than the equivalent activity would be if done indoors. There's also more evidence of the ways the physical environment can affect transmissibility outdoors. Much of why COVID is less transmissible outside has to do with air movement — so any circumstance that reduces natural airflow will affect risk, including geographic and meteorological conditions that temporarily reduce airflow in a given region. There is a lot of correlational evidence connecting high particulate matter, such as what happens during forest fires or on low-air-quality days, to COVID spread and severity. Researchers who have modeled airflow think that this might explain the correlations between COVID spread or severity and air pollution. It may not be the pollution itself making COVID worse, but the meteorological conditions that trap polluted air above a city could serve to trap more COVID-contaminated air as well. It's probably wise to think of air-quality alert days also as days where COVID may be more transmissible.
  • You can reduce the risk of indoor transmission with better ventilation by moving outside air through your space.
    Only five states have implemented ventilation standards for preventing COVID transmission in workplaces. But it has been known for years — long before the COVID pandemic — that increasing the movement of air through a building can drastically reduce the risk of contracting airborne pathogens. The amount of reduction varies by the kind of building, how many people are occupying it, and how much ventilation has been added. For example, doubling the airflow in an open-floor-plan office can reduce the relative risk of COVID infection by 44 percent. Most people don't have control over ventilation in their offices, but you can increase ventilation in your home without buying an expensive, new HVAC system. The World Health Organization recommends strategies like opening windows on opposite walls or using an outward-facing fan. When windows are closed, air conditioners and fans can actually end up increasing exposure to the virus by recirculating and mixing indoor air. The WHO recommends opening windows and doors for a few minutes every hour, and changing settings on central AC systems so they're pulling in more outdoor air. Exhaust fans in kitchens and bathrooms are also useful tools to help move air to the outside. Research has also suggested that changing furnace filters regularly and keeping the ducts in your AC or furnace clean can also help. It's also worth noting that in the wake of a federal judge striking down mask mandates on public transportation, airplanes are better ventilated while in flight than most stores and office buildings. There's still risk, for sure, especially in the airport itself or the plane before takeoff and after landing. But a plane’s ventilation does help reduce risk, even if you're the only one masking on the flight.