People Exposed to Coronavirus May Need to Take as Many as Three At-Home Tests, F.D.A. Says

The U.S. Food and Drug Administration issued a new recommendation on Thursday that asymptomatic people who are using coronavirus antigen tests take at least three tests, each spaced 48 hours apart, to reduce the odds of missing an infection.

People who have Covid-19 symptoms should take at least two tests, 48 hours apart, according to the agency.

The new guidelines come as the highly transmissible BA.5 subvariant of Omicron continues to spread, and after the Centers for Disease Control and Prevention eased its recommendation for routine surveillance testing in most circumstances.

Many people have reported that at-home tests failed to detect their infections, but studies have generally shown that rapid antigen tests are as good at detecting Omicron as they were at detecting Delta, the previous variant of concern.

The new recommendations are “very grounded in science,” said Dr. Michael Mina, a former Harvard epidemiologist who is now the chief science officer for eMed, which sells at-home tests. “Sometimes it takes the virus two days to grow to a detectable level and sometimes it takes six days to grow.”

Experts have long noted that rapid antigen tests, which are less sensitive than P.C.R. tests, are designed to be used serially, and that they are most likely to detect the coronavirus when people take them repeatedly over the course of several days.

The new recommendations emphasize the need for “additional testing over a longer period of time,” the agency said.

“The F.D.A.’s new recommendations for at-home Covid-19 antigen tests underscore the importance of repeat testing after a negative test result in order to increase the chances of detecting an infection,” Dr. Jeff Shuren, the director of the agency’s Center for Devices and Radiological Health, said in a statement.

The new guidance is based on the results of a new national study, which has not yet been published in a scientific journal. The study, led by researchers at the University of Massachusetts Chan Medical School, focused on 154 people who tested positive for the virus using P.C.R. tests between October 2021 and February of this year.

It found that among symptomatic people, two tests taken 48 hours apart detected 93 percent of infections. But the same testing pattern detected just 63 percent of infections in asymptomatic people.

When people without symptoms took three tests, each two days apart, the tests caught 79 percent of infections.

“We provide data-based evidence on how to test when using rapid antigen tests,” said Dr. Apurv Soni, an assistant professor at UMass Chan Medical School, who led the research. “The schedule of testing is important.”

Some people enrolled in the study had Delta infections, while others were infected with Omicron, the researchers said.

“The fact that the tests can detect Omicron is an important point that cannot be emphasized enough,” said Nathaniel Hafer, a molecular biologist at UMass Chan Medical School and an author of the study.

People who are worried that they may be infected even after receiving two or three negative results on at-home antigen tests can continue to test themselves, seek out a more sensitive P.C.R. test or consult with a doctor, the F.D.A. said.

Those who test positive using at-home tests, the agency said, should assume that they are infected and follow the guidelines set forth by the C.D.C.

The C.D.C. updated its Covid-19 guidance on Thursday but did not change its recommendation that people who test positive for the coronavirus isolate at home for at least five days.

People do not need to use the same brand of test each time, the F.D.A. said.

“If you plan to use at-home Covid-19 antigen tests, have several tests on hand so you can test more than once,” the agency said.