Pfizer on Wednesday released a preprint study showing its Covid-19 vaccine is slightly less effective at preventing symptomatic Covid-19 six months after the second dose, but is still highly effective at preventing severe Covid-19.

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Details on Pfizer's data

For the study, which began in July 2020, Pfizer followed 42,000 people in six countries for six months after their vaccinations.

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The study found Pfizer's vaccine was 96% effective at preventing symptomatic Covid-19 for the first two months following the second shot, then dropped in effectiveness by about 6% every two months after. Six months after the second dose, the study found the vaccine was 83.7% effective at preventing symptomatic infection.

However, the study found no decline in effectiveness against severe Covid-19. The vaccine remained 97% effective by that standard even six months after the second dose.

Natalie Dean, a biostatistician at Emory University, said Pfizer's data suggests that either that the immune response to the vaccine declines slightly over time, or that Pfizer's vaccine is simply less effective against emerging variants.

"It's some kind of modest decline, keeping it all in perspective that this number is still high," she said.

Will booster shots be needed?

Pfizer earlier this month announced that, in light of evidence suggesting a decline in vaccine effectiveness over time, it would seek FDA authorization for a third shot of its vaccine to serve as a booster. On Wednesday, it separately released data showing that a third shot drives a robust immune response.

Specifically, the new data shows that that antibody levels were five times higher against the delta variant following a third dose of its vaccine in 18- to 55-year-olds, and 11 times higher among 65- to 85-year-olds.

However, some experts argued that it's premature to consider booster shots, since Pfizer's current two-dose regimen remains so effective against severe Covid-19, and vaccines remain in short supply worldwide.

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Referring to the small decline in Pfizer's vaccine effectiveness, Akiko Iwasaki, an immunologist at Yale School of Medicine, said, "This drop is very slight—I wouldn't say it's waning." Iwasaki added that the study data doesn't "support a need for [booster shots] right now."

John Moore, an immunologist at Weill Cornell Medicine, cited numerous factors to consider related to booster shots. "There are scientific issues. There are ethical issues," he said. "What is the priority? In America, at the moment, it's getting doses one and two out to as many people as possible, particularly in undervaccinated regions."

Andrew Pavia, chief of pediatric infectious diseases at the University of Utah School of Medicine, advised a wait-and-see approach. "This is an area where everyone wants an answer before we have data," he said. "We really need to understand when we start seeing an increase in breakthroughs, an increase in serious breakthroughs, and which group that's occurring in first before we decide to give boosters."

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and chief medical adviser to the White House, emphasized that any booster shots will occur only after the conclusion of a regulatory process.

"As [acting FDA Commissioner] Janet Woodcock reminded everyone, we're talking about issues that are regulatory issues and public health recommendation issues, and Advisory Committee on Immunization Practices issues," Fauci said. "We're a long way from that. People are jumping the gun here."

For Pfizer's part, the company said it shares "a sense of urgency in staying ahead of the virus that causes Covid-19" with the federal government, but added the company "agree[s] that the scientific data will dictate next steps in the rigorous regulatory process that we always follow." (Anderson, Becker's Hospital Review, 7/28; Zimmer et al., New York Times, 7/28; Garde/Herper, STAT News, 7/28; Sullivan, The Hill, 7/28; Coleman, The Hill, 7/28; STAT News, 7/12)