By Sumathi Reddy

If you have had Covid-19, how long can you expect to be protected from another infection?

Doctors say the window between infections might be shrinking, fueled in part by the immune-evading Omicron BA.5 subvariant, although researchers are still gathering data.

The U.S. Centers for Disease Control and Prevention has long set 90 days as its window for what counts as a new Covid-19 infection—meaning that symptoms or positive Covid tests within 90 days of a prior infection have been considered the same infection. Getting reinfected sooner has always been possible but uncommon.

Now, reinfections are happening more often and can occur closer in time, say infectious-disease specialists and epidemiologists. They cite the march of new subvariants emerging and circulating simultaneously as well as BA.5’s ability to evade immune protections. In addition, doctors note, people are taking fewer precautions, such as masking indoors or avoiding large gatherings.

“This new variant’s superpower is reinfection,” says Peter Chin-Hong, a University of California, San Francisco, infectious-disease specialist and professor of medicine. The 90-day rule of thumb now is “completely out the window,” he says.

You are still unlikely to get reinfected with the same subvariant within a few weeks or months, doctors say. So if you just had BA.5, you probably won’t get it again this summer, they say.

The rapid rise of new variants and subvariants presents challenges. Most people won’t know for sure which subvariant they had, and many have been circulating in recent months. If you had a BA.2 infection a month ago, you might still be vulnerable to BA.5, scientists say, because the latter is better at evading immunity from vaccines or prior infections.

BA.5 is still new, and scientists lack definitive research about its reinfection rates, although medical experts around the world are revisiting earlier assumptions about reinfection windows.

Australian health officials recently changed their definition of what counts as a Covid reinfection to a new infection occurring at least 28 days after recovery from the last one, down from 12 weeks.

The CDC still defines a Covid reinfection as a new Covid infection occurring at least 90 days after a previous one, a definition that some doctors say is outdated. When the rule of thumb was first established, most people were getting diagnosed using PCR tests, which can detect dead virus and keep yielding positive results even when people are no longer infectious. The CDC didn’t respond to request for comment.

The 90-day rule took hold early in the pandemic when more people were taking precautions, such as social distancing, masking and avoiding travel, says John Wherry, director of the Institute for Immunology at the University of Pennsylvania. Now, many people who recover from Covid-19 resume normal life without taking preventive measures and get exposed again.

For people who were recently infected with a different Omicron subvariant, a new infection with BA.5 could happen in as little as four to eight weeks, UCSF’s Dr. Chin-Hong estimates, mainly because it can better evade immune defenses.

A Covid-19 infection is no longer a “get out of having Covid” card for the next three months. “I don’t think anyone should think they’re invincible,” Dr. Wherry says.

Even before BA.5 spread, research showed that reinfections within 90 days were possible, though uncommon.

Danish researchers found in a March preprint, which hasn’t been peer-reviewed, that though reinfections were rare, a number of them occurred within 60 days. Other preliminary studies, including one from the CDC, made similar findings.

A person’s vulnerability to reinfection increases over time, notes David Dowdy, an epidemiologist at Johns Hopkins Bloomberg School of Public Health in Baltimore.

“It’s not that after 90 days suddenly you become fully susceptible” to infection, he says. “It’s really that at 60 days you are somewhat susceptible and at 90 days you’re more susceptible and by 120 days you’re even more susceptible.”

An infection now with BA.5 will likely protect someone from reinfection by the same subvariant for perhaps roughly three or four months, he estimates, because that is the variant the body’s defenses will recognize best.

Some medical experts like Daniel Kuritzkes, chief of the division of infectious diseases at Brigham and Women’s Hospital in Boston, say the 90-day rule for reinfections is still reasonable. Some people will get reinfected sooner than two months, but he says most reinfections will happen after a longer window.

The two key factors, he says, are how a new variant differs from the one that infected a person previously and how persistent the person’s immune response has been after the most-recent infection.

He says a reinfection within two months is unlikely, but the outlook becomes fuzzier between two and three months. And as new variants emerge, the windows will continue to change.

“This is going to be a constantly changing parameter,” he says.