Tuesday, July 23, 2024

Rapid at-home tests can miss Omicron in early stages of infection

(HealthDay)—The Abbott BinaxNOW and Quidel QuickVue – two widely used rapid at-home COVID tests – may fail to spot evidence of the Omicron variant in the first days after infection, even when people are carrying substantial levels of the virus, preliminary research suggests. The researchers focused on 30 people infected with COVID at five workplaces that experienced what were most likely outbreaks of the Omicron variant in December. The people received both saliva-based PCR tests (the gold standard) and rapid antigen-based tests involving nasal swabs. It took three days, on average, for people to test positive on the two rapid antigen tests after their first positive PCR result, researchers reported. In four cases, people transmitted the virus to others after a negative result, according to the study, which has not been peer-reviewed. It is not yet clear whether the infections were missed because the antigen tests are inherently less sensitive to Omicron or because saliva tests may be better at detecting the new variant, The New York Times reported. One possible explanation? Omicron may replicate faster or earlier in the throat and mouth than in the nose, experts said. “While we’ll have to wait to see if the science bears out, that might be an indicator that that’s where the virus is growing first,” said Gigi Gronvall, an immunologist and testing expert at Johns Hopkins Bloomberg School of Public Health. “So if you’re going to look for the virus, which is what the tests do, then you may find more of it faster in the throat swab over the nose.” Reports have also surfaced that some people who initially tested negative on antigen tests when they swabbed inside their noses went on to receive a positive result when they swabbed the back of their throats. The latest study is 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 early days of infection. “The message is not that we should stop using these tests,” said Isabella Eckerle, a clinical virologist at the University of Geneva in Switzerland. But people should remain cautious after getting negative results, especially when they have symptoms or believe they may have been exposed to the virus. Why might the rapid tests be missing Omicron in the early stages of infection? They are designed to detect proteins lying on the surface of the coronavirus. If mutations in the virus change the structure of these proteins, antigen tests might miss the variant, experts said.  The study comes a week after the U.S. Food and Drug Administration released its own update on the effectiveness of the rapid antigen tests. “Early data suggests that antigen tests do detect the Omicron variant but may have reduced sensitivity,” the agency said in a statement. Many of those studies are early and small, and much more data is needed. The tests can deliver results at home in minutes and positive results are more reliable, scientists have said. That’s an important tool alongside PCR tests that can take days to come back. The Omicron variant has about 50 mutations, including more than 30 on the spike protein alone. Most rapid antigen tests are designed to detect more stable targets, the Times reported.

https://medicalxpress.com/news/2022-01-at-home-omicron-early-stages-infection.html

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