Emerging Data Raise Questions About Antigen Tests and Nasal Swabs

A new study suggests that the Abbott andQuidel at- home tests may not detect Omicron infections even when people have high levels of the coronaviruses.

The study focused on 30 people with the Omicron variant who were most likely to experience an outbreak in December. The people were given saliva-based P.C.R. tests and rapid antigen tests.

It takes three days for people to test positive for a rapid antigen test after a P.C.R. result. According to the study conducted by several members of the Covid-19 Sports and Society Working Group, in four cases people transmitted the virus to others while the rapid test showed the negative result.

It's not clear if the infections were missed because the tests are less sensitive to Omicron or because saliva tests are better at detecting the new variant.

The results are consistent with other preliminary evidence that the at- home tests that many Americans have come to rely on may fail to detect some Omicron cases in the first days.

The researchers said they shared their results with federal officials in real-time as the outbreak occurred last month.

The chair of the working group said that there were flaws with the testing.

The FDA released an update on the effectiveness of the rapid antigen tests a week ago. The agency said that early data suggests that the Omicron variant may have reduced sensitivity.

Many of the studies are small and early. Positive results are likely to be informative, and the tests that can deliver results at home in minutes remain an important public health tool. The wait for P.C.R. results can be long.

The message is not that we should stop using the tests, said a clinical virologist at the University of Geneva.

People should be careful when interpreting negative results, especially if they have symptoms or believe they have been exposed to the virus.

It is not a ticket that will allow you to go back to normal.

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There were vaccinations and testing at a subway station in New York last month.

Antigen tests can be used to detect coronaviruses. If the structure of the virus is changed, the tests will fail.

The Omicron variant has about 50 different changes. Most rapid antigen tests are designed to detect more stable targets.

John Moore, a virologist at Weill Cornell Medicine, said that even minor-seeming changes can affect a virus. Only experimental data can tell if there is an impact or not.

The F.D.A. told makers of rapid tests that they would have to continue to test their products as new versions of the same thing appeared.

Many companies have announced that their tests can detect Omicron, and several independent scientists said that the tests should be able to recognize the variant at high levels. Some new studies raise questions about the tests.

Bruce Tromberg, director of the National Institute of Biomedical Imaging and Bioengineering, said that the F.D.A. update was due to an evaluation they are doing with the National Institutes of Health. The scientists evaluated the tests using pooled samples of mucus from multiple people with Omicron, as well as similar pooled samples from Delta patients.

The scientists took the pooled samples and put them in a container. Dr. Tromberg said that the tests may be less sensitive to the new variant because they were able to detect more dilute solutions of Delta samples than Omicron samples. It may not translate into any different sensitivity in real-world settings.

A spokeswoman for the F.D.A. said last week that studies were underway to confirm the reason for the decreased sensitivity.

She said that adjustments to existing tests can be made by each developer with support from the F.D.A.

The F.D.A. update was the first hint that some tests may be less sensitive to Omicron. The performance of seven tests against samples of the Omicron virus grown from people with Omicron was evaluated by Dr. Eckerle and her colleagues. The tests were less sensitive to Omicron than previous versions.

They missed samples that had a decent viral load. The work has not been published.

Most of the tests that are not available in the United States have variability in their performance. An Australian study found that the tests had the same sensitivity to Omicron and Delta.

Experts said more data is needed.

David O'Connor, a virologist at the University of Wisconsin, Madison, urged people not to stop using rapid tests. It takes some time.

There were five New York and California workplaces that had a virus outbreak in December. In 29 of 30 people, samples of the virus produced a telltale genetic pattern, suggesting that they were most likely to have Omicron.

Each participant took P.C.R. and antigen tests over a period of up to 10 days. The first three days of the infection were the time when the results were most discordant.

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The global surge. The Omicron variant of the coronaviruses is less severe than earlier waves, and the last days of the year brought the good news. Governments are focusing more on expanding vaccination than limiting the spread.

Anne Wyllie is a microbiologist at the Yale School of Public Health and is an author of the new study. People should be aware of their high risk exposures. They shouldn't ignore the symptoms.

The other authors of the study are the founder of Infectious Economics and the chief executive of Mirimus Laboratories.

John Koval said that the finding that a P.C.R. test is more sensitive is not new.

He wrote in an email that frequent rapid testing is not realistic with P.C.R. due to the rapid nature of the test.

The president of the company, Douglas Bryant, said in a statement that the company has confirmed that its rapid tests are detecting the Omicron variant.

A company spokesman said that saliva-based P.C.R. tests were likely to detect the virus before nasal swabs if the study concluded that viral load peaks first in saliva. The company said that its rapid tests only have F.D.A. clearance.

The researchers found that the levels of virus peaked in saliva samples a day or two before they peaked in samples collected from the nose.

The findings are in line with a recent report from South Africa. The researchers found that the P.C.R. tests picked up a higher share of Omicron infections when saliva samples were used instead of nasal samples. The reverse was true for Delta.

The University of Cape Town's Diana Hardie said in an email that she would like to test both sites until we can compare the sensitivity of the tests.

One possibility is that Omicron replicates earlier in the throat and mouth than in the nose.

While we will have to wait to see if the science is correct, that could be an indicator that the virus is growing first, said Gronvall. If you look for the virus with the tests, you may find more of it in the throat swab over the nose.

Some people who tested negative on antigen tests when they were swabbed in their noses received a positive result when they were tested in the back of their throats, according to anecdotal reports.

Nathan Grubaugh is a researcher at the Yale School of Public Health. That warrants further investigation.

The United States does not allow the collection of saliva or samples from the throat for use in antigen tests.

Omicron has a rapid replication that means there is a narrower window to catch the virus before it begins spreading, and that people may need to test themselves sooner after a potential exposure and as close as possible to an upcoming gathering or event. When the tests are in short supply, they may need to test themselves more frequently.

Experts said that people who may have been exposed should take two tests over the course of two days.