Pfizer and BioNTech announced Thursday that they were developing a version of the coronavirus vaccine that targets delta, a highly contagious variant that has spread to nearly 100 countries. The companies expect to begin clinical trials of the vaccine in August.

Pfizer and BioNTech also reported promising results from studies of people who received a third dose of the original vaccine. A booster given six months after the second dose of the vaccine increases the potency of antibodies against the original virus and the beta variant by five- to tenfold, the companies said.

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Vaccine efficacy may decline six months after immunization, the companies said in a news release, and booster doses may be needed to fend off virus variants.

The data have not been published, nor peer-reviewed. The vaccine makers said they expected to submit their findings to the Food and Drug Administration in the coming weeks, a step toward gaining authorization for booster shots.

But the companies’ assertions contradict other research, and several experts pushed back against the claim that boosters will be needed.


“There’s really no indication for a third booster or a third dose of an mRNA vaccine, given the variants that we have circulating at this time,” said Dr. Céline Gounder, an infectious disease specialist at Bellevue Hospital Center in New York. “In fact, many of us question whether you will ever need boosters.”

Federal agencies also sounded a dubious note Thursday night. Generally, Americans who have been fully vaccinated do not need a booster shot at this time, the FDA and the Centers for Disease Control and Prevention said in a joint statement.

“We are prepared for booster doses if and when the science demonstrates that they are needed,” the agencies said.

The delta variant is believed to be about 60% more contagious than alpha, the version of the virus that tore through Britain and much of Europe earlier this year, and perhaps twice as contagious as the original coronavirus.

The delta variant is now driving outbreaks among unvaccinated populations in countries like Malaysia, Portugal, Indonesia and Australia. Delta is also now the dominant variant in the United States, the CDC reported this week.

Until recently, infections in the United States had plateaued at their lowest levels since early in the pandemic. Hospitalizations and deaths related to the virus have continued to decline, but new infections may be rising.

This article originally appeared in The New York Times.