The most transmissible variant yet of the coronavirus is threatening a fresh wave of infections in the United States, even among those who have recovered from the virus fairly recently.
The subvariant of Omicron known as BA.5 is now dominant, according to federal estimates released Tuesday, and together with BA.4, another subvariant, it is fueling an outbreak of cases and hospitalizations.
Though the popularity of home testing means reported cases are a significant undercount of the true infection rate, the share of tests that come back positive is shooting upward and is now higher than during most other waves of the pandemic. According to the C.D.C., the risk from Covid-19 is increasing in much of the country.
“I think there’s an underappreciation of what it’s going to do to the country, and it already is exerting its effect,” said Eric Topol, a professor of molecular medicine at Scripps Research, who has written about the subvariant.
BA.5 and BA.4, both subvariants of the Omicron variant that swept the world during the winter, are the most capable versions of the virus yet at evading immunity from previous infections and vaccines. Both variants have mutations in their spike proteins that are different enough from earlier versions of the virus that they are able to dodge some antibodies.
Waves of infection — and the subsequent immunity that comes with them — vary across countries and make for imperfect comparisons. Vaccination rates also vary. But in places where BA.4 and BA.5 have been dominant for weeks or months, the subvariants have caused increases in cases and hospitalizations, despite some population immunity from previous waves.
The C.D.C. says there is so far no evidence that BA.4 or BA.5 is inherently more severe than other Omicron subvariants, but when more people become infected, the number of people hospitalized because of the virus may also rise.
Prior infection with another form of the Omicron variant does confer some degree of immunity, Dr. Topol said, and that may explain why cases have not yet taken off sharply in the United States. “But it’s not anything like what we would hope,” he said. Omicron subvariants appear to represent a departure from earlier waves of the virus, when prior infection was more likely to protect against reinfection.
In Portugal, where vaccination rates are higher than in the United States, cases rose sharply after BA.5 became dominant in May, and hospitalizations neared their previous Omicron peak.
Before BA.4 and BA.5 became dominant in South Africa in April, research suggested that 98 percent of the population had some antibodies from vaccination or previous infection or both.
Even with those protective antibodies, many people in the country still became infected with BA.4 and BA.5, and the subvariants caused a small increase in cases, hospitalizations and deaths.
Places just emerging from significant spring waves of the virus may not be spared, either. Several countries in Europe had big outbreaks of a different Omicron subvariant, BA.2, that led to new waves of hospitalizations and deaths that peaked as recently as April. Yet in those countries, cases are ticking up again as BA.5 becomes dominant.
Experts say it is too early to fully predict what the latest subvariants may bring to the United States, which had an even more recent virus wave in May and June, driven by both BA.2 and yet another subvariant called BA.2.12.1. High shares of recent infections from BA.2.12.1 like that in the United States were uncommon among the countries now experiencing waves of BA.4 and BA.5 infections.
The United States also has a lower vaccination rate than many of those countries, and a much lower share of seniors who have had one or both booster shots.
“There’s a wave afoot, there’s no question about it,” Dr. Topol said. “My concern is the length of it.”
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