- The FDA said rapid antigen tests appear less sensitive against Omicron, per early lab studies.
- They are generally less accurate than PCR tests; which new data shows is more profound with Omicron.
- The tests are still helpful if used properly, experts previously told Insider.
Rapid antigen tests appear less sensitive to Omicron than previous variants, the Food and Drug Administration (FDA) said Tuesday.
"Early data suggests that antigen tests do detect the Omicron variant but may have reduced sensitivity," the FDA said.
The statement is based on laboratory findings, which are generally less dependable than clinical trial results.
But because clinical trials take a lot longer, laboratory findings are the "best way to evaluate true test performance in the short-term," per the FDA.
People can still use the rapid tests — which are cheaper and more convenient — but should be aware of their shortcomings, the FDA said.
A follow-up PCR test is more much reliable and should be used by people who have symptoms, or know they have been exposed to the coronavirus, per the FDA.
The news came as the White House prepares to distribute 500,000 free rapid COVID-19 tests and make over-the-counter kits free starting January.
How well the tests perform against Omicron has been debated widely as the new variant surged around the world.
The UK Health Security Agency previously said there was "no change in performance" of these tests against Omicron. This was also the position espoused by the FDA one week ago.
The latest run of experiments changed the agency's mind. In that batch , the FDA used live virus taken from patient samples, which is a better proxy for what is happening in the real world.
"Although the data is preliminary, the FDA believes it is important to share with the public," Stephanie Caccomo, an FDA spokesperson, told Politico.
"We will continue to keep the public informed if additional information comes to light."
Rapid tests are generally less sensitive
Even before this latest variant emerged, rapid tests were known to be hit and miss.
One review found that these tests were 58% accurate for those who didn't have symptoms and 72% accurate for those who did, Insider previously reported.
Timing of testing is also crucial, as it takes a couple of days for the virus to spread enough in the body to be picked up by a test, Insider's Hilary Brueck and Shayanne Gal previously reported.
The best bet, according to available data, is to take several rapid tests over a few days and test hours, not days, before mixing with others, experts previously told Insider.
That might be even more true for Omicron given the latest data, which suggests that the viral load required to trigger an antigen test is higher still with the variant.
Masking is still recommended in social settings, especially since vaccinated people seem more likely to carry Omicron than previous variants, per the CDC.
"Especially right now, with such high incidence across the country, if you feel any symptoms, assume you have Omicron," Michael Mina, an epidemiologist said on Tuesday, Insider previously reported.
"If it were me, and I were symptomatic starting today, I'd probably quarantine myself," he said, adding: "I would take the test tomorrow, and I would take the test in two or three days after that.