As Virus Grows Stealthier, Vaccine Makers Reconsider Battle Plans

As the coronavirus assumes contagious new forms around the world, two drug makers reported on Monday that their vaccines, while still effective, offer less protection against one variant and began revising plans to turn back an evolving pathogen that has killed more than two million people.

The news from Moderna and Pfizer-BioNTech underscored a realization by scientists that the virus is changing more quickly than once thought, and may well continue to develop in ways that help it elude the vaccines being deployed worldwide.

The announcements arrived even as President Biden banned travel to the United States from South Africa beginning on Saturday, in hopes of stanching the spread of one variant. And Merck, a leading drug company, on Monday abandoned two experimental coronavirus vaccines altogether, saying they did not produce a strong enough immune response against the original version of the virus.

Moderna and Pfizer-BioNTech both said their vaccines were effective against new variants of the coronavirus discovered in Britain and South Africa. But they are slightly less protective against the variant in South Africa, which may be more adept at dodging antibodies in the bloodstream.

The vaccines are the only ones authorized for emergency use in the United States.

As a precaution, Moderna has begun developing a new form of its vaccine that could be used as a booster shot against the variant in South Africa. “We’re doing it today to be ahead of the curve, should we need to,” Dr. Tal Zaks, Moderna’s chief medical officer, said in an interview. “I think of it as an insurance policy.”

“I don’t know if we need it, and I hope we don’t,” he added.

Moderna said it also planned to begin testing whether giving patients a third shot of its original vaccine as a booster could help fend off newly emerging forms of the virus.

Dr. Ugur Sahin, the chief executive of BioNTech, said in an interview on Monday that his company was talking to regulators around the world about what types of clinical trials and safety reviews would be required to authorize a new version of the Pfizer-BioNTech vaccine that would be better able to head off the variant in South Africa.

Studies showing decreased levels of antibodies against a new variant do not mean a vaccine is proportionately less effective, Dr. Sahin said....