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Here’s what you need to know about Omicron subvariants BA.4 and BA.5

By Martin Finucane Globe Staff ,Updated June 27, 2022, 2:18 p.m.

Experts are raising concerns about the rise of the BA.4 and BA.5 Omicron subvariants, saying they expect more people to catch cases of COVID-19 in the days ahead. Could the COVID-19 pandemic worsen?

Here’s what you need to know:

BA.4 and BA.5 are on the rise across the United States and in Massachusetts

BA.4 and BA.5 are being detected in a rising number of cases across the United States, according to the US Centers for Disease Control and Prevention. The newcomers are swiftly elbowing out the Omicron subvariants BA.2 and BA.2.12.1.

Researchers at the Broad Institute of MIT and Harvard estimate that as of mid-June, about half of COVID-19 cases in Massachusetts were being caused by BA.4 and BA.5, said Bronwyn MacInnis, director of pathogen genomic surveillance at the institute.

The researchers estimate that, as of Monday, about 77 percent of cases are being caused by BA.4 and BA.5 and the new subvariants will account for more than 90 percent of cases in the state by mid-July.

People are concerned because BA.4 and BA.5 seem to be better at reinfecting people

Experts say BA.4 and BA.5 appear to be better able to evade the immunity people have built up from getting sick from COVID-19 previously or getting vaccinated.

“The concern is that this appears to be another highly infectious variant that seems to be better at evading prior-infection immunity and vaccine-induced immunity than prior versions. So even infection with prior Omicron may not be protective,” Matthew Fox, a professor of epidemiology and global health at Boston University, said in an e-mail.

“That said,” he noted, “while it may be more severe than the prior Omicron variant (and we don’t have great data on that yet) it doesn’t appear to be much more severe and Omicron in general is less severe.”

Case increases in other countries have people worried

Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, said in a podcast last week that what other countries have experienced is sending worrisome signals.

He said several countries around the world have provided “real-world examples” of case surges happening at the same time as the BA.4 and BA.5 variants arrive.

“So it’s looking more and more certain that BA.4 and BA.5 will ultimately bring about another wave of cases over the weeks and months ahead,” he said.

“Multiple countries with high vaccination coverage (Israel, the UK, and others) are all showing upticks due to BA.4/BA.5; we will very likely see similar trends,” Andrew Lover, an assistant professor of epidemiology in the School of Public Health and Health Sciences at the University of Massachusetts Amherst, said in an e-mail.

MacInnis said the state’s current COVID-19 case numbers are “pretty stable and pretty low” now, but they could take a delayed turn upward thanks to BA.4 and BA.5, just as they did when previous variants came on the scene.

The impact’s not clear yet

Osterholm said, “I think one of the things that remains somewhat undetermined with BA.4 and the BA. 5 surges are the levels of severe disease and death that we might expect.”

“Make no mistake,” he said, if cases rise, hospitalizations and deaths will follow, “but the question is, how many hospitalizations and deaths will come as a result? And what does that mean for the health care system?”

Dr. Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center, told the Globe he anticipates the subvariants will spawn a summer of “substantial infections,” but low rates of hospitalization and death.

“If people have vaccine immunity or natural immunity, then they have substantial protection against severe disease,” Barouch said.

“On a population level, we expect our complex history of infection- and vaccination-induced immunity to continue to protect against severe disease,” said Katelyn Jetelina, a public health expert, last week in her widely read newsletter, “Your Local Epidemiologist.”

A new vaccine targeting BA.4 and BA.5 may or may not be on the way

Federal officials are pondering whether to update the existing COVID-19 vaccines to offer them in a booster shot to Americans in the fall.

One possibility is a so-called bivalent vaccine that would combine the existing formulation with one that targets BA.4 and BA.5, but there are several problems with that idea, including uncertainty about whether it would work, the time it would take to produce, and the possibility the virus might mutate further by the time the shot is going into people’s arms.

“Do we target BA.4 and 5? They will probably peak somewhere in the next month or two,” Dr. John Beigel, a clinical research director at the National Institutes of Health who has conducted multiple coronavirus vaccine studies, told The New York Times. “They may be old news by the time the fall comes.”

What should people do?

With the possibility looming of a new wave of cases, experts urged people to continue to take the oft-recommended precautions against getting COVID-19.

Dr. Sabrina Assoumou, an assistant professor of medicine at Boston University School of Medicine and an attending physician in the section of infectious diseases at Boston Medical Center, said, “It is critical that people get up to date with their vaccines.”

“If you’re anyone who qualifies for a booster, you should get it now,” she said. “If you haven’t been vaccinated and you have questions, talk to your health care professional.”

In addition, she said, “We need to use the other measures that we know that have been helpful in this pandemic.” She said people should get tested if they have symptoms, stay home if they’re sick, wear masks in indoor public places if they’re in a high-risk area, and — if they get sick — check with a health care provider about treatments.

Fox also emphasized the importance of vaccinations and boosting.

“Don’t panic, just prepare,” he said.

Globe correspondent Camille Caldera contributed to this report. Material from Globe wire services was used.