Rapid COVID-19 antigen tests may be less effective at detecting Omicron than earlier strains on the virus, the Food and Drug Administration said Tuesday.

The development was gleaned from preliminary studies conducted by the FDA in collaboration with National Institutes of Health’s RADx program into the performance of the at-home tests on the heavily-mutated Omicron variant.

“Early data suggests that antigen tests do detect the omicron variant but may have reduced sensitivity,” at detecting positive cases, the FDA said in a Tuesday statement.

Earlier testing was only done using heat-inactivated samples of Omicron. Using that method, the antigen tests detected Omicron with similar success to other variants.

“The FDA and RADx are continuing to further evaluate the performance of antigen tests using patient samples with live virus,” the FDA’s statement said.

The FDA said it is continuing to authorize use of the antigen tests.