As new variants of the virus spread, a race is on to vaccinate the public to protect against potentially more dangerous strains

A person took a photo of a mass COVID-19 vaccination site at Dodger Stadium in Los Angeles.Mario Tama/Getty Images via Bloomberg

As the coronavirus mutates and new, more contagious variants spread with alarming speed, the United States and the rest of the world are racing to vaccinate as many people as possible before potentially more dangerous strains emerge.

The overriding concern is that the continuing spread of COVID-19 — more than 1 million new infections every week in the United States alone — will allow the virus to evolve in more insidious ways that could defy the protections of existing vaccines, epidemiologists say.

“We’re absolutely racing to get prevalence down as quickly as we can, because we never know what the next mutation might be,” said Dr. Paul Biddinger, medical director for emergency preparedness at Mass General Brigham and chair of the state’s COVID-19 vaccine advisory group. “The more virus in the community, the more chances there are for the virus to mutate.”

Viruses naturally mutate as they proliferate, but the mutations are typically minor and don’t have a material impact on how they affect people. But when multiple mutations occur and the new virus succeeds in propagating, it’s considered a variant.

In recent months, variants of COVID-19 have been discovered in the United Kingdom, South Africa, and Brazil. They have raised growing concerns among epidemiologists because they appear to be more infectious and potentially more virulent, although the latter remains unclear.

Some preliminary evidence suggests the existing vaccines may provide less protection against the variants that have emerged in South Africa and Brazil, both of which were recently discovered in the United States. Those variants may also be able to infect people who previously contracted the dominant strain of COVID-19, which emerged in China.

On Friday, Johnson & Johnson announced that its closely watched COVID-19 vaccine prevented 66 percent of moderate and severe cases in a large clinical trial. But that number fell to 57 percent in South Africa, where the highly contagious variant is the dominant strain.

“It’s not the same pandemic as it was a few months ago,” said Dr. Dan Barouch, who runs the virology center at Beth Israel and is one of the world’s leading vaccine developers. “This is very strong efficacy data against a complex pandemic involving multiple resistant variants circulating globally.”

But even if the current vaccines don’t provide the same level of immunity against the new variants, they still appear to defend the body and are likely to reduce the severity of an infection, said Erin Bromage, a biology professor who studies infectious diseases at the University of Massachusetts Dartmouth.

For example, if a person requires a certain level of antibodies to fight off the dominant strain of the virus, he or she may need five times that to fight off some of the new variants, he said. But the vaccines are likely to generate significantly more antibodies than the body needs to defend itself.

“There seems to be enough of a buffer to provide sufficient protection,” Bromage said.

That said, Moderna, Pfizer, and other vaccine producers have been studying the variants to determine whether they need to adjust their vaccines or prepare additional booster shots to increase their effectiveness.

The greater concern is what could come next, as the virus spreads like wildfire across the planet. This week, the virus passed another milestone, having infected more than 100 million people, a quarter of them in the United States.

On Thursday, health officials said the variant identified in South Africa had been found in the United States for the first time, with two cases diagnosed in South Carolina. Neither of those infected had any history of traveling or any connection to each other, officials said, suggesting the variant has been spreading through the community.

“I am worried,” said Dr. David Hamer, an infectious disease specialist at Boston Medical Center. “There’s a lot of disease being transmitted, and that sets the stage for vaccine-resistant mutations. If we had better control, that would reduce the opportunity for the rapid evolution of the virus.”

For that reason, he and others said, it’s too soon to relax restrictions and they questioned plans announced by Mayor Martin J. Walsh Tuesday to allow Boston fitness centers, movie theaters, museums, and other indoor recreational venues to reopen.

Most businesses will remain subject to the 25 percent capacity limit, which the state extended through Feb. 8, and other gatherings will remain capped at 10 people indoors and 25 people outdoors.

Officials in Boston justified their decision by noting that the number of cases in the city has fallen in recent days. Earlier this month, the number of infections exceeded a seven-day average of 7,500 active cases, but that has since dropped to about 6,500.

“We’ve seen a little bit of improvement in our cases,” Marty Martinez, the city’s chief of health and human services, said at a City Hall news conference. “We’ve seen a decrease in hospitalizations. The state has seen that across the Commonwealth, and we are seeing that in the city of Boston. And so, it was a decision to sort of get to a place where we can start to reopen again.”

But the numbers remain unacceptably high, some epidemiologists say, urging government officials to do more to reduce the spread of the virus.

“I was flabbergasted to learn about the reopening,” said Dr. Christopher Gill, an associate professor of global health and infectious disease specialist at the Boston University School of Public Health. “This is not the time to be easing up.”

Everyone should continue to wear masks, remain physically distant from others — especially indoors — and continue washing their hands regularly, he and others said. Those basic measures remain vital safeguards against new variants, but the best protection against the prospect of more dangerous strains is for as many people to get vaccinated as soon as they can.

In the United States, that likely means as much as 80 percent of the population will have to be immunized, said William Hanage, an associate professor of epidemiology at the Harvard T.H. Chan School of Public Health.

“There needs to be a firebreak to the wildfire of the virus,” he said.

Jonathan Saltzman of the Globe staff contributed to this report.