Pfizer will test a booster shot and develop an adapted vaccine to address variant concerns.

As concerns grow that new coronavirus variants could blunt the protective effects of vaccines, Pfizer and BioNTech said on Thursday that they planned to test a third booster shot as well as update their original vaccine.

Laboratory experiments have found that the levels of antibodies neutralized by the Pfizer-BioNTech shot was reduced against a variant first identified in South Africa, which could hint at reduced efficacy. But there is no evidence yet from clinical trials there suggesting that the vaccine does not offer strong protection.

“We are taking multiple steps to act decisively and be ready in case a strain becomes resistant to the protection afforded by the vaccine,” Dr. Albert Bourla, the chief executive of Pfizer, said in a statement.

One study will look at what kind of protection is given when people receive a third shot about six to 12 months after the initial two-dose regimen. In addition, the companies said they were speaking to regulators about testing an adapted version of the vaccine that would protect against the variant from South Africa, known as B.1.351.

Moderna, which developed a vaccine using the same technology as the Pfizer-BioNTech product, said on Wednesday that it had shipped doses of a newly adjusted vaccine to the National Institutes of Health for testing. The adapted vaccine also addresses the B.1.351 variant, which seems to dampen the effectiveness of the existing vaccines.

Dr. Phil R. Dormitzer, Pfizer’s vice president and chief scientific officer of viral vaccines, said in an interview that the companies believed that a third shot, even in the original formulation, could strengthen the body’s immune system in fending off virus mutations.

The companies’ announcements come the same week that the Food and Drug Administration released draft guidance to the drug industry about adapting vaccines to address new variants. The agency said that companies would not have to undergo the same large-scale clinical trials that led to the original vaccines.

Dr. Gregory Poland, the director of the Vaccine Research Group at the Mayo Clinic in Rochester, Minn., said that Pfizer and other companies were smart to prepare for the possibility that a variant might make their vaccines less effective. He added that more information on how the original vaccines work against new variants was needed before fully switching gears.

“A variant can change in days and a completely different one can take over,” he said. “You’ve got to have good epidemiological evidence and good real-world effectiveness data to know is a variant vaccine worthwhile, and in whom.”

Because Pfizer and BioNTech believe their existing vaccine still offers good protection against the variants, Dr. Dormitzer said, their adapted vaccine plan is more like a test run — a way of learning how to quickly create and study a tweaked vaccine in case a more threatening mutation develops in the future.

“The thought behind the test is not that we think we need to change the vaccine right now — we think we probably do not,” Dr. Dormitzer said.

But, he said, the companies could apply lessons they learn from the clinical trials they are planning “if we find ourselves in a situation in the future where we really had to move fast because a new strain started circulating that was not covered by the vaccine.”

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