Why antigen tests may still work well for Omicron, despite "reduced sensitivity" in lab studies
Early research backed by the National Institutes of Health on several popular rapid COVID-19 antigen tests suggest the kits will still work to detect cases of the Omicron variant despite a "reduced sensitivity" seen in early lab studies with some brands, federal health officials said Wednesday.
Scientists backed by the NIH have been racing to run experiments to assess Omicron's impact on the performance of antigen tests, compared to previous strains of the virus. The rapid tests, which can be done at home, have surged in popularity amid a record spike in COVID-19 cases this month.
The Food and Drug Administration said Tuesday that some antigen tests may have "reduced sensitivity" for Omicron, citing early data from the NIH's laboratory studies. Lower sensitivity means there might be a greater chance of missing traces of the virus and giving false negative results. However, NIH scientists caution that their lab finding is not evidence of a significant drop in the real-world performance of popular at-home tests, even in the "worst case scenario."
"A change in sensitivity in the laboratory is not a guarantee that there's a change in sensitivity from a clinical point of view. I would not recommend that people suddenly abandon their antigen tests," says Bruce Tromberg, director of the National Institute of Biomedical Imaging and Bioengineering.
Tromberg heads the NIH's RADx program that spans a large swath of the Biden administration's effort to ramp up COVID-19 testing. The initiative has worked to pave the way for the FDA greenlighting new tests, including shepherding the recent authorizations of over-the-counter tests from Siemens and Roche. A task force backed by the program has also been assessing variants of concern since last January, studying how they might undermine tests authorized in the U.S..
Through the program, scientists at Emory University have been gathering samples collected from Omicron patients around the country. Those samples are pooled together in their lab and then gradually diluted until off-the-shelf tests can no longer spot the virus in the mixture.
Compared to similar experiments the lab has run previously, Tromberg said scientists initially found antigen tests were still able to detect cases of Omicron roughly as well as previous variants using "heat inactivated" samples. But later, when studying various antigen test brands using pools of "live virus," some could only detect the virus in more concentrated mixtures of Omicron compared to the Delta variant.
Enough data to better measure whether that change is meaningfully affecting the "clinical" performance of antigen tests — their real-world effectiveness at detecting the virus in people — could be gathered within weeks, Tromberg guessed. Beyond the NIH and FDA's efforts, researchers from test manufacturers, universities, and health authorities abroad are also scrutinizing the ability of antigen tests to spot Omicron cases in the real world.
"It could be that the sensitivity in the clinic has dropped. It could be that the sensitivity is the same. It could even be better. We certainly have seen that as well. So it's still super early to say and we need to fill out that picture," said Tromberg.
The companies behind several at-home test options, including the Abbott BinaxNOW, Quidel QuickVue, Ellume, and Roche SD Biosensor tests, have recently issued statements saying that data suggests their tests will perform similarly for Omicron as with previous variants.
A spokesperson for Access Bio said they were still working on submitting updated performance evaluations for their CareStart test to the NIH, but that their products can detect Omicron with "identical sensitivity when tested with recombinant protein" of the variant.
Experts say other factors could also impact antigen testing's ability to spot Omicron beyond the lab, ranging from where the new variant first infects the body — for example, in the upper respiratory tract versus the lungs — to signs it may have a shorter incubation period and lead to higher loads of virus. Repeating the test two days in a row also improves the odds antigen tests can spot infections.
"When you're dealing with an antigen test, everyone knows from the beginning that it is not, by the nature of the technical aspect of the test, as sensitive as a PCR," Dr. Anthony Fauci, the president's chief medical adviser, told reporters on Wednesday.
Federal health officials have urged Americans to seek out COVID-19 tests before holiday gatherings, even if they have received a booster shot, as an added way to curb Omicron's unprecedented spread. The pace of new cases is at record highs nationwide and could be enough to overwhelm hospitals in some regions of the country, despite signs Omicron poses a smaller risk of severe disease.
"The fact that the sensitivity is diminished somewhat does not obviate the importance of the still advantage and usefulness of these tests under different circumstances," said Fauci.