The effectiveness of the Pfizer-BioNTech Covid vaccine waned over six months, but experts say the data still don't point to an immediate need for booster shots.
The study, which hasn't yet been peer-reviewed or published in a medical journal, found that the vaccine was 97 percent effective at preventing severe disease from Covid-19 for at least six months — but the effectiveness against any symptomatic illness fell from 96 percent to 84 percent in the same period, falling by about 6 percent every two months.
"I was generally encouraged by the results of the paper," said the lead study author, Dr. Stephen Thomas, a coordinating investigator for the Pfizer vaccine trial and director of the SUNY Upstate Institute for Global Health & Translational Science in New York.
He said the expectation was always that the vaccine's protection was going to wane. The big question, he said, was whether it would wane to a degree that would affect the so-called public health burden of the disease, specifically hospitalizations and deaths. So far, that doesn't appear to be the case.
"Even though we saw that at six months there was a waning of protection, there was a maintenance of protection against those severe outcomes that really make up the public health burden of the disease," Thomas said.
Overall, the study found that the vaccine was 91 percent effective at preventing any symptoms of Covid-19, from mild to severe, over six months. Pfizer announced that finding in a news release in April.
Dr. Paul Offit, a vaccine researcher at Children's Hospital of Philadelphia, said he was "positively surprised" that the effectiveness was as high as it was for preventing symptoms.
"This data is all really encouraging and exactly what you'd expect," he said.
"You're never going to be able to protect as well against asymptomatic infection or mildly symptomatic infection, and that's OK," he said. "You just want to keep people out of the hospital and keep them from dying. That's the goal."
The new research looked at follow-up data gathered through March 13 on the more than 44,000 people who participated in Pfizer's Phase 3 clinical trial last year. People received either two doses of the vaccine or two doses of a placebo three weeks apart. Because the study looked at data only through mid-March, it remains unclear how the shots fare against the delta variant, which became the most common strain of the virus in the U.S. early this month.
Experts continue to consider whether booster shots will be necessary. Pfizer CEO Albert Bourla has long said a booster would be needed in the coming months, although federal health officials say there's not yet evidence to say it's warranted.
In an earnings call Wednesday morning, Pfizer said a third dose increased levels of antibodies specific to the delta variant fivefold in people ages 18 to 55 and elevenfold in those 65 to 85. Although the findings suggest better protection against the variant, more research is needed to determine whether that translates to better protection against illness.
Offit stressed that because the vaccine remains very effective at preventing severe disease and hospitalizations, it's too early to definitively say booster shots will be required.
"If the numbers climb to 5, 10 or 20 percent of people who are fully vaccinated and are still hospitalized or killed, then you can start thinking about a booster, but we're not there yet," he said.
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Dr. Bob Wachter, chairman of the department of medicine at the University of California, San Francisco, agreed that the new data don't suggest it's time to rush out and get booster shots yet.
"This just shows that someone who has got their shots seven or eight months ago is at a little bit higher risk" for a breakthrough infection "than we thought, and it's a perfectly good justification for even vaccinated people to go back to wearing masks indoors," Wachter said, referring to the Centers for Disease Control and Prevention's decision Tuesday to recommend that vaccinated people wear masks indoors in areas with high levels of community spread.
But the dominance of the delta variant, combined with waning effectiveness of the vaccine over time, suggests that boosters will be needed at some point, he said.
"I don't think it's grounds for panic, and I don't think it's grounds to run out and find a booster today, because the overall protection against getting super sick and dying remains extraordinarily high," Wachter said. "But I do think it is one piece of the puzzle that says to all of us boosters are going to be in our future."
Thomas, the study author, said more research is needed.
"I think [the study] says that there is a possibility that we may need booster doses if the protective efficacy of the vaccine continues to decline over time," Thomas said. "But that to me is still an unanswered question."