What We Know About Why Some People Never Get Covid 19

Americans who haven’t had covid-19 are now officially in the minority. A study published this week from the US Centers for Disease Control and Prevention (CDC) found that 58% of randomly selected blood samples from adults contained antibodies indicating that they had previously been infected with the virus; among children, that rate was 75%.

What is different about that minority of people that hasn’t yet gotten infected? Stories abound of close calls, of situations where people are sure they could have (or should have) gotten sick, but somehow dodged infection. Not all the questions are answered yet, but the question of what distinguishes the never-covid cohort is a growing area of research even as the US moves “out of the full-blown” pandemic. Here are the possibilities that scientists are considering to explain why some people haven’t contracted the virus.

They behave differently

We’ve seen it play out time and time again—some people adhere more strictly to protocols known to reduce transmission of the virus, including wearing a mask and getting vaccinated. Some people avoid large public settings and may have even been doing so before the pandemic, says Nicholas Pullen, a biology professor at the University of Northern Colorado. Then again, that doesn’t tell the whole story; as Pullen himself notes: “Ironically, I happen to be one of those ‘never COVIDers’ and I teach in huge classrooms!”

They’ve trained their immune systems

The immune system, as any immunologist or allergist can tell you, is complicated. Though vaccination against covid-19 can make symptoms more mild for some people, it can prevent others from contracting the illness altogether.

Growing evidence suggests that there may be other ways that people are protected against the virus even without specific vaccines against it. Some could have previously been infected with other coronaviruses, which may allow their immune systems to remember and fight similarly shaped viruses. Another study suggests that strong defenses in the innate immune system, barriers and other processes that prevent pathogens from infecting a person’s body, may also prevent infection.

An innate immune system that’s already not functioning as well due to other medical conditions or lifestyle factors such as sleep or diet may put a person at higher risk of getting sick from a pathogen. There’s not single answer here yet, but initial studies are intriguing and may offer avenues for future treatments for covid-19 and other conditions.

They’re genetically different

In the past, studies have found interesting associations between certain genetic variants and people’s susceptibility to communicable diseases such as HIV, tuberculosis, and the flu. Naturally, researchers wondered if such a variant could exist for covid-19. One June 2021 study that was not peer reviewed found an association between a genetic variant and lower risk of contracting covid-19; another large-scale study, focused on couples in which one person got sick while the other didn’t, kicked off in Oct. 2021.

“My speculation is that something will be borne out there, because it has been well observed that resistance embedded in genetic variation is selected in pandemics,” Pullen says. But most experts suspect that even if they are able to identify such a variant with some certainty, it’s likely to be rare. For now, it’s best for those who haven’t gotten covid to assume they’re as susceptible as anyone else. Whatever the reasons some people haven’t yet gotten sick, the best defense remains staying up to date with vaccinations and avoiding contact with the virus.

Source: What we know about why some people never get covid-19 — Quartz

“Being exposed to the SARS-CoV-2 virus doesn’t always result in infection, and we’ve been keen to understand why,” study author Rhia Kundu said in a statement, using the scientific name for the coronavirus. “We found that high levels of pre-existing T cells, created by the body when infected with other human coronaviruses like the common cold, can protect against COVID-19 infection.”

The study, which examined 52 people who lived with someone who contracted the coronavirus, found that those who didn’t get infected had significantly higher levels of T cells from previous common cold coronavirus infections. T cells are part of the immune system and believed to protect the body from infection. “Our study provides the clearest evidence to date that T cells induced by common cold coronaviruses play a protective role against SARS-CoV-2 infection,” study author Ajit Lalvani said in a statement.

Researchers cautioned that the findings should not be relied upon as a protection strategy. “While this is an important discovery, it is only one form of protection, and I would stress that no one should rely on this alone,” Kundu said. “Instead, the best way to protect yourself against COVID-19 is to be fully vaccinated, including getting your booster dose.” And the findings on the subject have been inconsistent, with other studies actually suggesting that previous infection with some coronaviruses have the opposite effect.

A major question that has come from the so-called ‘never COVID’ group is whether genetics plays a role in preventing infection. In fact, the question has spurred a team of international researchers to look for people who are genetically resistant to COVID-19 in the hopes that their findings could improve therapeutics. “What we are doing essentially is that we are testing the hypothesis that some people might not be able to get infected because of their genetic and inborn makeup, meaning that they might be genetically resistant to COVID,” says Spaan, who is a member of the COVID Human Genetic Effort.

The effort has sequenced genetic data from about 700 individuals so far, but enrollment is ongoing and researchers have received thousands of inquiries, according to Spaan. The study has several criteria, including laboratory test confirmation that the person has not had previous COVID-19 infection, intense exposure to the virus without access to personal protective equipment like masks and an unvaccinated status at the time of exposure, among others. So far, the group doesn’t know what the genetic difference could be – or if it even exists at all, though they believe it does.

“We do not know how frequent it is actually occurring,” Spaan says. “Is it like a super rare individual with a very, very rare mutation? Or is that something more common?” But the hypothesis is “embedded in human history,” according to Spaan. “COVID is not quite the first pandemic that we are dealing with,” Spaan says. “Humans have been exposed to viruses and other pathogens across time from the early beginning, and these infections have left an imprint on our genetic makeup.”

Those who haven’t gotten the coronavirus are “very much at risk,” says Murphy of Northwestern University. “I think every unvaccinated person is going to get it before this is over.” Experts stressed that research to determine why some people get COVID-19 while others don’t is still very much underway, and no one should rely on any of the hypotheses for protection. Instead, those who haven’t gotten the coronavirus should continue mitigation measures that have been proven to work, like vaccination and mask-wearing.

“You don’t ever want to have COVID,” Murphy says. “You just don’t know which people are going to get really sick from this and die or who’s going to get long COVID, which is hard to diagnose and difficult to treat and very real.” But with coronavirus cases on the rise and mitigation measures like mask mandates dropping left and right, it’s not an easy task.

COVID19: Face masks could return as cases spike Financial Mirror

06:48 Tue, 21 Jun
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Fourth Shot Protects Against Severe Omicron Outcomes; COVID May Increase Risk of Rare Eye Blood Clots

The following is a summary of some recent studies on COVID-19. They include research that warrants further study to corroborate the findings and that has yet to be certified by peer review.

Fourth vaccine dose protects vs Omicron for at least a month

A fourth dose of the COVID-19 vaccine from Pfizer and BioNTech provided significant added protection against severe disease, hospitalization and death for at least a month in older individuals, according to a study conducted when the Omicron variant was dominant.

The estimated effectiveness of the fourth dose during days 7 to 30 after it was administered compared with a third dose given at least fourth months earlier was 45% against infection, 55% for symptomatic disease, 68% for hospitalization, 62% for severe disease and 74% for death, the research team reported on Wednesday in The New England Journal of Medicine.

The study compared 182,122 individuals aged 60 and older who received a fourth dose and 182,122 very similar people who had received a third dose but not a fourth.

“The results of our real-world study suggest that a fourth vaccine dose is, at least initially, effective against the Omicron variant,” the researchers said. “Additional follow-up will allow further assessment of the protection provided by the fourth dose over time.”

A recently published study that looked only at rates of breakthrough infections and serious illness after the fourth dose found that efficacy waned quickly versus infection but held steady versus severe illness.

COVID-19 may increase risk for rare eye clots. Patients with COVID-19 may have an increased risk of rare vision-threatening blood clots in the eye for months afterward, new findings suggest.

Because SARS-CoV-2 infections increase the risk of blood vessel obstructions at other sites in the body, researchers studied nearly half a million COVID-19 patients to see whether they would develop clots in the veins or arteries of the retina, the nerve tissue at the back of the eye that receives images and sends them to the brain.

Over the next six months, 65 patients had a retinal vein occlusion. While that number is low, it reflects a statistically significant 54% increase compared with pre-COVID infection rates, according to a report published on Thursday in JAMA Ophthalmology.

Retinal artery clots were 35% more common after COVID-19 than before, but that difference might have been due to chance. The clots most often occurred in patients with other conditions that increased their risk of blood vessel problems, such as diabetes, high blood pressure, and high cholesterol.

By: Nancy Lapid

Source: Fourth shot protects against severe Omicron outcomes; COVID may increase risk of rare eye blood clots

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Critics:

By: Mary Van Beusekom

Compared with a third vaccine dose, a fourth dose of the Pfizer/BioNTech COVID-19 vaccine lowered the risk of infection, symptomatic infection, hospitalization, severe illness, and death 52% to 76%—depending on the measure—amid the Omicron surge among older adults.

Protection against infection waned, however, after 5 weeks, but not protection against severe COVID-19. The findings were published yesterday in the New England Journal of Medicine.

Seven to 30 days after the fourth COVID-19 dose, vaccine effectiveness (VE) relative to the third dose was estimated at 45% against infection (95% confidence interval [CI], 44% to 47%), 55% against symptomatic illness (95% CI, 53% to 58%), 68% against COVID-19 hospitalization (95% CI, 59% to 74%), 62% against severe disease (95% CI, 50% to 74%), and 74% against death (95% CI, 50% to 90%).

Fourteen to 30 days after the fourth dose, VE was 52% (95% CI, 49% to 54%) against infection, 61% (95% CI, 58% to 64%) against symptomatic illness, 72% (95% CI, 63% to 79%) against hospitalization, 64% (95% CI, 48% to 77%) against severe disease, and 76% (95% CI, 48% to 91%) against death.

In the fourth week after the fourth dose, the adjusted infection rate was lower by a factor of 2.0 (95% CI, 1.9 to 2.1) than that in the three-dose group and lower by a factor of 1.8 (95% CI, 1.7 to 1.9) than that among controls.

The difference in absolute risk for COVID-19 hospitalization 7 to 30 days after a fourth vaccine dose, relative to a third, was 180.1 per 100,000 people (95% CI, 142.8 to 211.9), while it was 68.8 cases per 100,000 (95% CI, 48.5 to 91.9) for severe disease. A sensitivity analyses of VE against infection had similar results as those in the primary analysis.

Starting in the fifth week after the fourth dose, the rate ratio (RR) for infection began to fall. The adjusted rate of infection in the eighth week after the fourth dose was comparable to that of internal controls. The RR for the three-dose group relative to the four-dose group was 1.1, while the rate ratio for the internal control group, compared with the four-dose groups, was 1.0.

The RRs comparing controls with fourth-dose recipients were larger and lasted longer for severe disease. In the fourth week after the fourth dose, the adjusted rate of severe disease was lower by a factor of 3.5 than in three-dose recipients and a factor of 2.3 than in internal controls.

The adjusted rate of severe illness in the fourth week after the fourth dose was 1.6 cases per 100,000 person-days, compared with 5.5 cases per 100,000 in three-dose recipients and 3.6 cases per 100,000 in internal controls. The adjusted rate differences were 3.9 fewer cases per 100,000 person-days and 2.1 fewer cases per 100,000 than the three-dose group and internal controls, respectively.

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World First Study Infecting Volunteers With COVID Delivers Initial Results

The first findings from a landmark study following healthy adults deliberately infected with SARS-CoV-2 have revealed new and unexpected insights into the earliest stages of COVID-19. The data indicates the virus’s incubation period is shorter than expected and rapid antigen tests are incredibly effective at identifying people when they are most infectious.

In what are known as “human challenge studies,” scientists have intentionally infected healthy adults with pathogens for well over a century. Although ethically controversial, human challenge studies have informed medical innovations for a number of viral infections, including cholera, typhoid and influenza.

Early in 2020, soon after the novel coronavirus SARS-CoV-2 emerged, some researchers began calling for human challenge studies to help accelerate insights into this new virus. Progress on SARS-CoV-2 challenge studies was slow as many were cautious to approve this kind of research. It wasn’t until the beginning of 2021 that the first human challenge trial commenced, and now one year later we are finally seeing preliminary data from those initial studies.

This first study covers 35 healthy volunteers aged between 18 and 29 with no history of previous SARS-CoV-2 infection. The trial utilized a sample of one of the first strains of SARS-CoV-2 taken from a patient early in the pandemic before more concerning variants emerged.

Because this was the first SARS-CoV-2 human challenge trial, an extremely low dose of the virus was tested. A single drop of viral material was administered nasally to all volunteers. The amount of virus used in the study was 10 times lower than the amount recommended by an independent advisory panel.

Despite the extraordinarily low dose administered, the researchers still saw 53 percent of the cohort developing a PCR-confirmed SARS-CoV-2 infection. Of the 18 positive infections in the study, only two presented with no symptoms.

However, those two asymptomatic subjects consistently displayed significant viral levels in their upper airways similar to those symptomatic subjects. This suggests asymptomatic infections can be as infectious as symptomatic infections.

Interestingly, the study found that the time from initial viral exposure to first symptoms was on average only 42 hours. This is significantly shorter than the three-to-five-day incubation period initially calculated for this original strain of SARS-CoV-2.

The virus was first detected in the throats of volunteers before moving up to the nose at around the three-day point after exposure. Viral levels were seen to peak at higher volumes in the nose compared to the throat, leading the researchers to indicate shedding from the nose to be a greater threat to others than shedding from the throat.

“With virus present at significantly higher titres in the nose than the throat, these data provide clear evidence that emphasizes the critical importance of wearing face coverings over the nose as well as mouth,” the researchers write in the study.

Of the 16 symptomatic subjects in the study, a variety of expected symptoms were detected, including headache, sore throat, malaise and rhinitis. Seven subjects developed fever and around half the symptomatic cohort experienced complete loss of smell.

Five symptomatic subjects experienced signs of long COVID, primarily a loss of smell that persisted for over six months. At a 12-month follow-up all symptoms had dissipated in all subjects.

One of the most immediately valuable findings in the study was the correlation between positive rapid antigen test results and high viral loads. Chief investigator on the trial Christopher Chiu says these tests, known as lateral flow tests in the United Kingdom, are very effective at identifying when a person is most infectious with this virus.

“We found that overall, lateral flow tests correlate very well with the presence of infectious virus,” said Chiu. “Even though in the first day or two they may be less sensitive, if you use them correctly and repeatedly, and act on them if they read positive, this will have a major impact on interrupting viral spread.”

Chiu pointed out that, although this study was conducted using the original strain of the virus, the findings still offer important insights into the nature of SARS-CoV-2 infections that will apply to more recently circulating variants.

“While there are differences in transmissibility due to the emergence of variants, such as Delta and Omicron, fundamentally, this is the same disease and the same factors will be responsible for protection against it,” said Chiu. “From the point of view of virus transmission related to the very high viral loads, we are likely if anything to be underestimating infectivity because we were using an older strain of the virus. With a newer strain, there might be differences in terms of size of response, but ultimately we expect our study to be fundamentally representative of this kind of infection.”

Moving forward, perhaps the biggest outcome from this first-of-its-kind study is the demonstration of how challenge studies with SARS-CoV-2 can be safely conducted. Doug Jones, from the British Society of Immunology, says these kinds of challenge studies will be increasingly important for the development of COVID-19 therapeutics and vaccines in the future.

“This is the first step in developing human challenge studies on COVID-19,” said Jones. “While the main aim of this study was to establish a safe and successful protocol to build on in the future, the significance of it should not be underestimated. In the longer-term, the hope is that these findings will now open up a new research avenue to develop a platform that will allow us to speed up the development of new vaccines, antivirals and diagnostics against COVID-19.”

The new study is yet to be peer-reviewed and published in a journal but it is available as a preprint on Research Square.

How Omicron Upended What We Thought We Knew About Natural Immunity

After dizzily swelling for weeks, COVID-19 cases seem to be leveling off in New York and Chicago. In the greater Boston area, the amount of SARS-CoV-2 found in wastewater is going down as quickly as it had gone up. The hard part isn’t over yet, but the omicron wave is starting to break and roll back out to sea. Soon we’ll see if any treasures are left behind in the tide pool.

Between Dec. 1 and Jan. 17, at least 18 million Americans contracted COVID. Data suggests that the vast majority of those cases were in unvaccinated people, but plenty of people who got their primary series of the vaccine also caught the immunity-evading omicron variant. By the time this wave is over, American bodies will know this virus like never before. But will the survivors gain anything from having had the disease? After all, there will be more variants in the future. Could the hard-earned immunity we’ve gained from omicron help fight them off? Could this wave be the last?

On Monday, White House chief medical adviser Anthony Fauci said it’s too soon to answer these questions. Scientists we spoke to agreed. But they also said the reason these questions were so difficult to answer was because of an issue that hasn’t always gotten much attention in the public sphere:

The immunity provided by a COVID infection itself. Scientists have learned a lot about this “natural immunity” since the pandemic began. But omicron has upended many of those expectations, and the more we learn about this variant, the less clear it is what we should expect for the future of the virus and our immunity to it.

Scientists have been studying infection-induced immunity since COVID first emerged. In fact, it was the only kind of immunity anyone could really study at that point, said John Moore, a professor of microbiology and immunology at Cornell University’s Weill Cornell Medical College. And while there are now many more studies on vaccine-induced immunity thanks to clinical trials and easily trackable vaccinated populations like medical staff, there’s a lot that can be said about natural immunity, pre-omicron, with a reasonable amount of certainty.

One important takeaway from all that pre-omicron research: Infection-induced immunity and vaccine-induced immunity are pretty similar. On the whole, studies found that the efficacy of infection-induced immunity was about the same as what you’d get from a two-dose mRNA vaccine, and sometimes higher.

For example, research from the U.K., in which a few hundred thousand participants were followed in a large-scale longitudinal survey, found that prior to May 16, having had two doses of the vaccine (regardless of the type) reduced the risk of testing positive by 79 percent, while being unvaccinated and having had a previous infection reduced the risk by 65 percent. After the delta variant became dominant,1 vaccination became less effective, reducing the risk by 67 percent, while a previous infection reduced the risk by 71 percent.

Likewise, both kinds of immunity seemed to wane over time — though Moore said infection-induced immunity might take longer to decline because a vaccination happens nearly all at once, while an infection takes longer to go through a process of growing, declining and finally being cleared from the body. “But it’s also not radically different [from antibody titers to vaccination]. It’s not measured in years, but months,” he said.

This is why some countries, including the member states of the European Union, treat documented recovery from COVID-19 as functionally the same as vaccination in their “vaccine passport” systems.

Still, vaccine-induced immunity is a better choice, not because it produces a stronger immunity, but because it enables you to get the immunity without the side effects and risks that come along with illness — like a greater risk of stillbirth if you’re pregnant, or long COVID, hospitalization and death in general.

The pre-omicron research also indicated another downside to natural immunity: namely, that it can be more variable. All immunity differs from person to person and holds up better against some variants than others. But infection-induced immunity can also be more or less effective depending on how severe your case of COVID was, explained John Dennehy, a professor of biology at the City University of New York’s Graduate Center.

Since the earliest studies, scientists have found evidence that more severe illnesses produce a higher antibody response, while mild cases end up producing much less.

Then came omicron. The public desire for information on omicron is moving faster than science can produce, but we do know that this variant escapes natural immunity as easily as it does vaccine immunity. Omicron carries a lot of mutations that make it able to evade antibodies — and it doesn’t really matter how you got those antibodies in the first place, said Jeffrey Klausner, a professor of medicine in the Division of Infectious Diseases at UCLA’s David Geffen School of Medicine.

Beyond that, the picture is murky. For example, we know milder infections have, with past strains, produced less effective immunity. If a hallmark of omicron is milder infections — and that’s the main reason why there’s so much chatter that it might just be better to get this variant and get some natural immunity — how much immunity can anyone really expect to come out of those mild infections with?

“We’re going to know for sure in a few weeks because a ton of preprint is coming out about it, but I don’t know the answer today,” Moore said. It’s information journalists can come back and update you on later, but it makes informed speculation hard now. (Meanwhile, keep an eye on our COVID-19 research tracker.)

The same holds true when you start trying to parse out what vaccinated people can expect from a breakthrough case of omicron. The combination of vaccine and infection-induced immunity has been shown to produce a hybrid that is probably more effective than either type alone — but, again, that research came from pre-omicron studies. Is a breakthrough case as good as a booster?

If you’re going to get a booster after you’ve had a breakthrough case, how long should you wait? Those are questions scientists don’t have the answers to yet, partly because there’s no clear through line of what to expect once you’re dealing with omicron.

“Maybe your readers are right in being confused, because we don’t really know how long-lasting the immunity you get from omicron will be,” said David Thomas, the director of the Division of Infectious Diseases at Johns Hopkins Medicine.

Which brings us to the biggest question of all: Will the many infections, reinfections and breakthrough infections associated with omicron maybe — finally — put us in a better position for a well-protected, safer society? Maybe even a society that doesn’t have any more big waves crashing on its head?

Theoretically, yes, Klausner told me. And he’s optimistic that it will. Thomas and Dennehy, on the other hand, were more cautious. After all, Dennehy pointed out, there’s no guarantee that future strains will be related to omicron. If omicron is different enough from delta that it evades immunity from that previous variant, what happens if a future variant comes along that’s evolved from delta and not omicron? It’s not unreasonable to expect a whole new wave.

And what does Moore think? He was just ready to take a pause from speculation and get some data before anyone starts making decisions for themselves or for society. “I’m fed up with winging answers to reporters like yourself, because I don’t know the answer,” he said. “None of us know for sure.”

Maggie Koerth

By:

Maggie Koerth is a senior science writer for FiveThirtyEight.

Source: How Omicron Upended What We Thought We Knew About Natural Immunity | FiveThirtyEight

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Do Rapid Tests Work With Omicron? Should I Swab My Throat? Covid Test Questions Answered

Should we be swabbing our noses or our throats for at-home tests? Do rapid tests even detect omicron at all? Are PCR tests the only results we can trust right now?

Guidance about how to approach testing in the omicron era seems to be evolving by the day. ​​A recent real-world study that followed 30 subjects likely exposed to omicron found that PCR saliva tests can catch Covid-19 cases three days before rapid antigen tests, which use nasal swabs.

These findings, which have not been peer reviewed, follow the Food and Drug Administration’s announcement in late December that, while they do detect omicron, rapid antigen tests may now have “reduced sensitivity.” But that doesn’t mean rapid tests don’t play a key role in our pandemic response going forward.

This is all confusing to a public that’s been pulled in several directions over the course of the pandemic when it comes to guidance and testing. Long delays for PCR test results, a shortage of at-home rapid tests, and the wait for more definitive science about the omicron variant have all made it more difficult to figure out when and how to to get tested. Nevertheless, public health experts say that, as more become available, rapid tests will be an increasingly vital tool for diagnosing Covid-19 and reducing its spread.

“We don’t want the perfect to be the enemy of the good”

So you might be wondering: What’s the point if rapid tests aren’t as accurate as PCR tests? Well, rapid antigen tests, which look for a specific protein on the Covid-19 virus, remain extremely effective at confirming positive cases. Put simply, if you test positive on a rapid test, you almost certainly have Covid-19.

If you test negative, in some cases, you might still test positive on a PCR test, which is much more sensitive because it tests for genetic evidence of the virus. Rapid tests may not pick up positive cases in people who have been vaccinated or who have recently recovered from Covid-19, since they may produce less virus, one expert told Recode.

Rapid tests can also reveal a positive case faster than the labs that process PCR tests, since they can take several days to share results with patients, especially during big waves of infection. Perhaps more importantly, rapid tests can indicate whether someone is contagious enough to spread the virus to others, which is what many people are most worried about.

“Given that a rapid antigen test is often the most feasible or available option for many, we don’t want the perfect to be the enemy of the good,” Joshua Michaud, the associate director for global health policy at the Kaiser Family Foundation, told Recode. He explained that every Covid-19 case that’s caught by someone who could take a rapid antigen test but not a PCR test is a win for public health.

Taking rapid tests more frequently also makes them more effective. Most at-home rapid test kits are designed to be conducted over the course of two days, which is why kits typically include two tests. Because each test is a snapshot of the moment it’s taken, multiple tests help reduce the chance of receiving a false negative.

Of course, all of this is assuming that you can get your hands on a rapid test. In the weeks since omicron started to spread, rapid tests have been incredibly hard to find in some parts of the country. These tests are out of stock because neither test manufacturers nor the Biden administration anticipated record levels of Covid-19 cases, which have boosted the demand for rapid tests.

To confront the shortage, the White House plans to buy and distribute 500 million free rapid tests in the coming weeks. When that happens, these tests could help catch more positive cases and lower the number of people infected with Covid-19.

How accurate are rapid tests when it comes to omicron?

The accuracy of a rapid test depends on how often you’re testing yourself and whether you want to identify a Covid-19 infection or measure your contagiousness. But if you test positive on a rapid test, you should trust the result, assume you’re infectious, and isolate for at least five days. If you test positive again after five days, the CDC recommends isolating for five more.

Rapid tests, however, are not perfect. Research indicates that antigen tests are less accurate than PCR tests — this has been the case since the beginning of the pandemic. PCR tests are processed in a lab, where sophisticated equipment can identify and amplify even the tiniest genetic evidence of the virus that causes Covid-19.

These tests are so precise that patients can actually test positive for weeks after they’ve recovered and are no longer contagious. The results of rapid tests, meanwhile, can vary based on how much virus is in a patient’s nose at the time the sample is taken and how far along they are in their infection.

Scientists explain the difference between rapid tests and PCR tests in two ways: specificity, which reflects a test’s false-positive rate, and sensitivity, which reflects a test’s false-negative rate. Both PCR and rapid tests have high specificity, which means that their positive results are very trustworthy. But while PCR tests tend to have near-perfect sensitivity, rapid antigen tests tend to have a sensitivity around 80 to 90 percent. This means that rapid tests tend to produce more false negatives than PCR tests do.

“Most at-home tests are still able to detect infection by omicron because they target a part of the virus that doesn’t mutate that much”Omicron makes testing even trickier. The sensitivity of rapid tests may be even lower for omicron cases, according to early research from the FDA and other scientists.

Another problem is that omicron may propagate more in the throat than the lungs, and it could take longer for Covid-19 to show up in nasal samples, even if someone is symptomatic. It’s possible that vaccinated people and people who have recently recovered from Covid-19 are noticing more false negatives on rapid tests because they tend to produce less virus overall.

“At-home tests are mostly effective when the person has high viral loads, a time when the person is more likely to transmit the virus,” Pablo Penaloza-MacMaster, a viral immunologist at Northwestern’s medical school, told Recode, “Most at-home tests are still able to detect infection by omicron because they target a part of the virus that doesn’t mutate that much.”

Separate studies from both the UK’s Health Security Agency and researchers in Australia found that antigen tests are as sensitive to the omicron variant as they were to earlier strains of Covid-19. Again, the FDA does still recommend rapid tests to diagnose positive cases, and test manufacturers say they’re confident in their products’ ability to detect omicron.

While early research indicates saliva tests might detect Covid-19 more quickly, right now most of the PCR tests and all of the available rapid at-home tests that have emergency use authorizations from the FDA use nasal samples.

How to use rapid tests in less-than-ideal circumstances

Which brings us back to the question of whether you should be sticking nasal swabs in your throat. There is evidence that saliva samples may be a quicker indicator of Covid-19 cases, but that doesn’t mean you should stop following the directions that come with your test kit.

The FDA says that people should not use rapid antigen tests to swab their own mouths. Some experts say you might consider doing so anyway, and point out that other countries, including the UK, have approved rapid antigen tests that use throat swabs and released very careful directions about how to do so.

“​​I personally do swab my throat and my nose to get the best sensitivity when I use over-the-counter tests at home,” Michael Mina, an epidemiologist at Harvard, said at a Thursday press conference. “There are risks associated with that, but the biology does tell us that they might be getting better sensitivity earlier.”

But the concern with rapid test kits right now is not that people are swabbing their noses, but how often they’re swabbing their noses. A single test could miss a Covid-19 case and produce a false negative, but taking two tests over a 24 to 36 hour period reduces this risk.

The more rapid tests you take, the more you reduce your chances of a false negative, and the more times you test negative over multiple days, the more confident you can be that you’re not spreading Covid-19.

Still, the biggest problem right now is that rapid tests are pricey and hard to find. Pharmacies have limited the number of test kits people can buy, and many are completely sold out. A single test can also cost more than $10, which means that testing yourself regularly gets expensive quickly. Opportunists have even hoarded tests and engaged in price gouging, which has exacerbated the shortage.

If you don’t have enough tests to test yourself regularly, it’s best to test yourself right before seeing vulnerable people, says Mara Aspinall, a professor who leads Arizona State’s testing diagnostic commons and a board member for the test manufacturer Orasure, told Recode. “I’m heading to a vulnerable person [or] I’m going into a health care setting, and therefore need to test right beforehand.”

For now, the best test kit is the test you can get (Wired has a handy list of the brands currently available). If you’re planning to go somewhere and don’t want to spread the virus, you should take one rapid test the day before traveling, and then a second test immediately before you go. If you only have one rapid test, take it right before you see people.

Testing yourself should become easier as more rapid tests become available. In addition to the 500 million free rapid tests that the White House will distribute beginning later this month, people with private insurance will also be able to get their rapid test purchases reimbursed starting next week. You should also check with your local health department, as they might be distributing free tests.

Even though the rapid test situation is still less than ideal, there are other strategies we can use to protect both ourselves and other people from Covid-19, like getting vaccinated, getting boosted, and wearing a mask. And if you do happen to find some rapid tests, go ahead and grab them. They might just come in handy, especially if you use them correctly.

Correction, January 7, 10:30 am: An earlier version of this story misstated in one instance the kind of false results that might appear more often on rapid Covid-19 tests among vaccinated people and those with immunity from recent infection. The false results are false negatives, not false positives.

Rebecca Heilweil

Rebecca Heilweil is a reporter for Open Sourced, covering emerging technologies, artificial intelligence, and logistics. Her Twitter handle is @rebheilweil.

Source: Do rapid tests work with omicron? Should I swab my throat? Covid test questions, answered. – Vox

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