Rapid tests may not detect omicron early in infection

Preliminary evidence suggests that rapid tests for COVID-19 may not be able to detect the omicron variant during the first few days of the disease.

The study looked at 30 people from five different places in New York and California who tested positive for the disease in December of 2021. Due to their workplace policies, each person was undergoing both daily rapid tests and daily PCR tests, which take longer to process but can detect smaller amounts of virus. The Abbott andQuidel rapid antigen tests are approved by the FDA for use in the study.

The team noted in their report that all but one of the participants were likely to have the omicron variant.
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On the day of their first positive test, the rapid tests all came back negative. It took nearly two days after the positive test for any of the rapid tests to come back positive. The median time from first positive to first positive was three days, according to the researchers.

In 28 of the 30 cases, the amount of virus detected by PCR was high enough to cause other people to get sick. The team confirmed that in four of the cases, the people who had the virus passed it on to others while still being tested negative for it.
The lead author of the study, Blythe Adamson, told STAT News that there were many more transmissions than four. Four of them were confirmed through contact tracing and epidemiology investigation. There were probably many more.
The New York Times reported that early data and anecdotal reports suggest that the tests could be more sensitive if they were done in the throat. This idea needs further research. If you have a negative result on a rapid test, the study suggests isolating if you have any COVID-19 symptoms.
A negative rapid test result is not a ticket that will allow you to go back to normal or drop any other measures, according to the Times. If a person has been tested negative for a rapid test but has symptoms that are consistent with the COVID-19 virus, they should exercise caution.

The new study has not yet been peer reviewed, but its results are in line with a recent update from the FDA. The update states that early data suggests that the omicron variant may have reduced sensitivity, based on the agency's own lab studies. Bruce Tromberg, director of the National Institute of Biomedical Imaging and Bioengineering, told The New York Times that reduced sensitivity in the lab doesn't always translate to reduced sensitivity in real-world applications.
The new preprint suggests that the FDA's reduced sensitivity may translate to real-world test results, creating a lag between positive and negative results. Rapid tests can still detect the omicron variant, they just take longer than the PCR.
Abbott spokesman John Koval told the Times that frequent rapid testing is not realistic because of the rapid nature of the test. At- home rapid tests are more readily available. People should be wary of false-negative results, test themselves at least twice over two days, and keep a record of their symptoms regardless of their test results, the Times reported.

Why are the rapid tests less sensitive to the omicron variant? The coronaviruses's surface can be detected with a rapid antigen test, but as the virus changes, it can become less recognizable to the test. The FDA spokeswoman told the Times that the FDA can support the adjustment of existing tests if necessary.

The FDA-approved rapid tests are only approved for use in the nose, not the throat or mouth. Preliminary studies have shown that omicron can replicate faster in the mouth and throat than in the nose. The authors of the medRxiv paper found the same results in an analysis of five people who took both nose and saliva-based tests. They found that the amount of virus in the saliva of these individuals peaked one to two days before the nose.
The major unknown is whether the rapid antigen tests are inherently less able to detect omicron, or if there is less omicron to detect on nasal swabs. John Moore is a professor of immunology at Cornell. The answer to that question is not known.
The New York Times has a story about the new study.
Live Science published the original article.