People exposed to the coronavirus may need as many as three home tests, the FDA says

The US Food and Drug Administration issued a new recommendation on Thursday that asymptomatic people using coronavirus antigen tests take at least three tests, each 48 hours apart, to reduce the chances 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 cases.

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

The new recommendations are “very grounded in science,” said Dr. Michael Mina, a former Harvard epidemiologist who is now chief science officer for eMed, which sells home tests. “Sometimes it takes two days for the virus 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 PCR tests, are designed to be used serially and are most likely to detect the coronavirus when people take them repeatedly over several days.

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

“The FDA’s new recommendations for home tests for the Covid-19 antigen emphasize the importance of repeating testing after a negative test result to increase the chances of detecting an infection,” Dr. Jeff Shuren, 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 PCR tests between October 2021 and February this year.

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

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

“We provide data-based evidence of 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 test schedule 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 author of the study.

People who are concerned that they may be infected even after receiving two or three negative results on antigen tests at home can continue to test themselves, seek a more sensitive PCR test or consult a doctor, the FDA said.

Those who test positive using home tests, the agency said, should assume they are infected and follow guidelines provided by the CDC

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

People don’t have to use the same type of test every time, the FDA said.

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