“Americans who are fully vaccinated currently do not need a booster,” the statement said, adding, “We are prepared for booster doses when and when science shows they are needed.”

The move can make economic sense for Pfizer-BioNTech. Since the outbreak of the pandemic, the partner companies have been following a “get to market first” strategy in the manufacture and marketing of their vaccines.

The companies did not accept federal funds or participate in Operation Warp Speed, former President Donald J. Trump’s fast-track vaccine initiative. Not only were they the first to get Food and Drug Administration approval for their coronavirus vaccine, the first to use the novel mRNA technology, but also the first to get their vaccine approved in adolescents.

The strategy has “paid off as well as you could wish,” said Steve Brozak, president of WBB Securities, a biotechnology-focused research investment bank.

Last week, Pfizer and BioNTech said a booster given six months after the second dose of the vaccine increased the effectiveness of antibodies against the original virus and beta variant by five to ten times. But antibody levels may not be the best biological measure of need for booster doses, say experts, who say it’s no surprise that antibodies increase after a third dose.

“The antibody response is not the only measure of immune protection,” said Dr. Leana S. Wen, a former health commissioner for Baltimore. “There have been several studies to suggest that these vaccines also stimulate B-cell and T-cell immunity. Even if there aren’t that many antibodies, it doesn’t mean someone isn’t protected. “

In Israel, the government has agreed to provide Pfizer with data on its vaccine recipients, and Pfizer has cross-checked the Israeli data with its own laboratory tests for antibody responses. Some people familiar with the data say that the two sets of data taken together suggest that immunity in those vaccinated wears off after about six to eight months, leading to an increasing number of breakthrough infections.