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Pfizer says vaccine's power wanes over time. Are you still protected?

New research from Pfizer found a drop-off in protection after six months, though the vaccines remained highly effective against severe illness.

The effectiveness of the Pfizer-BioNTech COVID-19 vaccine waned over six months, but experts say the data still don’t point to an immediate need for booster shots.

The study, which has not yet been peer-reviewed or published in a medical journal, found that the vaccine was 97% effective at preventing severe disease from COVID-19 for at least six months — but the effectiveness against any symptomatic illness fell from 96% to 84% in the same time period, falling roughly 6% every two months.

“I was generally encouraged by the results of the paper,” said lead study author Dr. Stephen Thomas, who is 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 the protection would wane to a degree that would affect the so-called public health burden of the disease; namely, 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 the vaccine was 91% effective at preventing any symptoms of COVID-19, ranging from mild to severe, over the course of six months. Pfizer announced that specific finding in April in a press release.

Dr. Paul Offit, a vaccine researcher at the 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. Individuals either received two doses of the vaccine or two doses of a placebo three weeks apart. Because the study only looked at data 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 United States in early July.

The findings come as experts continue to consider whether or not booster shots will be necessary. Pfizer CEO Albert Bourla has long said that a booster would be needed in the coming months, though federal health officials say there’s not yet evidence to say it’s warranted.

In an earnings call Wednesday morning, Pfizer said that a third dose increased levels of antibodies specific to the delta variant by fivefold in people ages 18 to 55, and by elevenfold in ages 65 to 85. Though these findings suggest better protection against the variant, more research is needed to determine whether this 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 that booster shots will be required.

“If the numbers climb to 5, 10, or 20% 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.

Dr. Bob Wachter, chairman of the department of medicine at University of California, San Francisco, agreed with Offit that the new data do not suggest it’s time to rush out and get a booster shot 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 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 in time, 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.”

This story was originally published on NBC News.