Omicron may not be detected early in infection by rapid assays

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According to a new study, PCR testing could detect the virus days before fast screenings.

In the early stages of infection, when a person is shedding the virus in sufficient numbers to be contagious, preliminary evidence suggests that rapid antigen testing for COVID-19 may fail to detect the omicron version consistently.

All 30 patients who tested positive for SARS-CoV-2 at five different New York and California workplaces in December 2021 were studied by researchers in the new preprint database medRxiv, which was published Wednesday (Jan. 5). They had daily quick tests and daily PCR testing performed on each employee because of workplace policies; the PCR tests took longer to run but could identify lesser levels of virus. Abbott BinaxNOW and Quidel QuickVue fast antigen tests, approved by the Food and Drug Administration, were the tests used in the study (FDA).

A genetic quirk of the omicron variation showed up on PCR tests in all but one of the patients, the team concluded in their study.

A day after each person's first positive PCR test, all of their fast tests were negative, according to STAT News (opens in new tab). The fast tests didn't appear positive for anything for nearly two days following the positive PCR. According to the study's findings, it took three days on average for all individuals: "the median time from first positive PCR to first detectable antigen positive."

This even though in 28 of the 30 instances, PCR results showed that the virus had the potential to spread on Day 1. The study discovered that in four of those cases, infected people transmitted the virus to others even while they remained negative for the virus on quick testing.

An epidemiologist at Infectious Economics in New York City and a Flatiron Health staffer tells STAT News that it's "certainly possible" that there were more than four transmissions. "We named four because we were able to trace and investigate four cases of contact tracing and epidemiology. There were probably a lot more than that."

In addition to swabbing the nose, additional early evidence and anecdotal accounts suggest that swabbing the throat may increase the test's sensitivity earlier in infection, according to The New York Times. Further research is required to confirm this hypothesis. A negative result on a fast test does not negate the necessity of isolation in the meanwhile, according to the findings of this study.

According to Isabella Eckerle, a clinical virologist at the University of Geneva in Switzerland, "a negative fast test result is not a ticket that allows you to return to normal or drop any other measures," she said. People who have tested negative on a fast test but exhibit symptoms compatible with COVID-19 or believe they have been exposed to the virus should use extra caution.

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Even though the new study hasn't been peer-reviewed, its findings align with the FDA's latest upgrade. Early evidence suggests that antigen tests detect the omicron form but may have reduced sensitivity, according to the agency's lab investigations, the update states. According to National Institute of Biomedical Imaging Director Bruce Tromberg, decreasing sensitivity in the lab doesn't usually translate into reduced sensitivity in real-world applications.

Preprints suggest that FDA's reduced sensitivity may translate to real-world test findings, producing a delay between positive results from fast testing and positive PCR tests, as suggested by the recent preprint. However, this does not rule out the use of quick tests to identify the omicron variation; they take longer than PCR.

"Frequent fast antigen testing does reduce transmission—and with a highly infectious variation, frequent testing is needed, which is not realistic with PCR," Abbott spokesperson John Koval told the New York Times. While at-home quick testing is commonly available, PCR tests are not. According to the Times, people should be aware of false-negative findings, test themselves at least twice over two days following exposure, and keep track of any suspected COVID-19 symptoms.

It's important to ask now: why is the omicron variety less susceptible to fast tests? It is possible that proteins on the coronavirus's surface can become less identifiable to the test as the virus mutates. Identifying and correcting these problematic mutations, FDA spokesperson Stephanie Caccomo told the Times, "may be done by each developer, if appropriate, with cooperation from the FDA."

The FDA-approved rapid tests, however, are only approved for use in the nose and not the throat or mouth, so this could be another Achilles heel for rapid tests. Times writes that anecdotal and preliminary studies suggest that omicron may grow more quickly in the mouth and throat than in the nose. The authors discovered similar results in an analysis of five participants who took both nose swab- and saliva-based PCR tests in the study by the medRxiv paper. They discovered that in these people, the virus in their saliva peaked one to two days before the virus in their noses.

Is there less omicron to detect on nasal swabs because of the lower sensitivity of [quick antigen testing], or is there less omicron to detect in the first place? According to Weill Cornell Medical College microbiologist and immunologist John Moore, speaking to STAT News. That question is still unanswered at this time.

The new medRxiv study can be read about in STAT News and the New York Times.

Reference : https://www.livescience.com/rapid-test-accuracy-omicron-coronavirus-infections

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Early in an infection, rapid testing could fail to detect omicron.

Rapid testing may miss omicron early in infection.

Rapid testing may not detect omicron in the early stages of an illness.

There is a possibility that rapid testing will not discover omicron at an early stage of infection.