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The efficacy of the Covid-19 vaccine developed by Pfizer and BioNTech fell from 96% to 84% over six months, according to data released Wednesday, a decline that could fuel Pfizer’s case that a third dose will eventually be required.

The data, released in a preprint that has not been reviewed by outside scientists, suggest the vaccine was 91% effective overall at preventing Covid-19 over the course of six months.

In the ongoing study, which enrolled more than 44,000 volunteers, the vaccine’s efficacy in preventing any Covid-19 infection that causes even minor symptoms appeared to decline by an average of 6% every two months after administration. It peaked at more than 96% within two months of vaccination and slipped to 84% after six months.

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Against severe disease, which includes people with low blood oxygen levels or who are hospitalized, the overall efficacy of the vaccine was 97%.

To Paul Offit, a pediatrician and vaccine expert at Children’s Hospital of Philadelphia, the results are “very reassuring.” The potential need for booster shots is tied to the number of fully vaccinated people who develop severe disease, Offit said. Of the more than 23,000 volunteers who received the vaccine, only one came down with severe Covid-19 within six months, suggesting two doses of Pfizer’s vaccine offers adequate protection, he said.

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While the data suggest Pfizer and BioNTech’s vaccine provides lasting protection against the worst symptoms of Covid-19, the paper leaves open the possibility that booster doses will eventually be necessary to curtail infection. If the vaccine’s efficacy continues to decline at the rate observed in the paper, it would fall below the 50% threshold — a benchmark for vaccine utility — within 18 months of vaccination.

That would support Pfizer’s contention that two doses of the vaccine won’t be enough to provide long-term protection. Federal authorities have maintained that people who have been fully vaccinated don’t need booster doses yet but that they are continuing to look at new data.

Natalie Dean, a biostatistician at Emory University, said the data point to two possibilities. Either the vaccine’s efficacy against mild disease is decreasing slightly, or the emergence of new viral variants made it look less effective. “It’s some kind of modest decline, keeping it all in perspective that this number is still high.” She also emphasized that as more data are collected, the vaccine remains effective at preventing severe disease.

Moderna’s vaccine was 90% effective against symptomatic Covid-19 and 95% effective against severe disease after six months, the company said in an April press release. Johnson & Johnson has not yet disclosed six-month efficacy data.

The Pfizer study, which enrolled volunteers in Europe and the Americas, doesn’t address whether the vaccine might be less effective against the fast-spreading Delta variant. Pfizer and BioNTech have conducted lab studies suggesting the vaccine should be able to neutralize the variant, but there is no large-scale clinical data to confirm that conclusion.

Just over half of the study’s participants were male. About 82% of participants were white, 9.5% were Black, 4.3% were Asian, 1% were Native American. Roughly 26% of volunteers identified as Hispanic.

Dean said that it’s important to remember that we are still learning about these new vaccines, and that different vaccines may still turn out to have advantages and disadvantages.

“For me it’s just remembering why it’s so important that we maintain a wide profile of vaccines and exploring a mix and match approach and also the intranasal vaccines.” Dean said. “We still don’t know what is ultimately going to be the best strategy. We want this wide portfolio because durability is always a question and so is robustness against different variants. This only represents six months of data early on in the life of these vaccines. There are open questions.”

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