Omicron’s large number of mutations have raised the possibility that it could evade protections against Covid that had come through vaccines and treatments like some monoclonal antibodies. The FDA noted the presence of mutations in the virus can diminish test performance depending on the sequence of the variant, the design of the test and the prevalence of the variant in the population.

Many popular at-home antigen tests are intended to be used serially — meaning people should take two tests on separate days — to ensure more accurate results.

FDA reemphasized its recommendation that the public “closely follow the tests’ instructions,” adding that if people test negative on an antigen test but have symptoms of Covid-19 or have a “high likelihood of infection due to exposure,” they should seek out a molecular test to confirm the result.

“Although the data is preliminary, the FDA believes it is important to share with the public,” FDA spokesperson Stephanie Caccomo told POLITICO. “We will continue to keep the public informed if additional information comes to light.”

The new NIH data comes less than a week after a different NIH study of heat-inactivated Omicron samples found antigen tests were able to detect the Omicron “with similar performance” to other variants.

The Biden administration recently announced it intends to purchase 500 million at-home tests and begin distributing them in January. But it is unclear how much supply of the popular rapid tests will be available immediately: the government’s solicitation for the tests was posted last week and U.S. manufacturing levels remain limited.