A series of studies found that the Moderna vaccine seemed to be more protective as the months passed than the Pfizer-BioNTech vaccine. Here’s why.

By Apoorva Mandavilli

It was a constant refrain from federal health officials after the coronavirus vaccines were authorized: These shots are all equally effective.

That has turned out not to be true.

Roughly 221 million doses of the Pfizer-BioNTech vaccine have been dispensed thus far in the United States, compared with about 150 million doses of Moderna’s vaccine. In a half-dozen studies published over the past few weeks, Moderna’s vaccine appeared to be more protective than the Pfizer-BioNTech vaccine in the months after immunization.

The latest such study, published on Wednesday in The New England Journal of Medicine, evaluated the real-world effectiveness of the vaccines at preventing symptomatic illness in about 5,000 health care workers in 25 states. The study found that the Pfizer-BioNTech vaccine had an effectiveness of 88.8 percent, compared with Moderna’s 96.3 percent.

Research published on Friday by the Centers for Disease Control and Prevention found that the efficacy of the Pfizer-BioNTech vaccine against hospitalization fell from 91 percent to 77 percent after a four-month period following the second shot. The Moderna vaccine showed no decline over the same period.

If the efficacy gap continues to widen, it may have implications for the debate on booster shots. Federal agencies this week are evaluating the need for a third shot of the Pfizer-BioNTech vaccine for some high-risk groups, including older adults.

Scientists who were initially skeptical of the reported differences between the Moderna and Pfizer-BioNTech vaccines have slowly become convinced that the disparity is small but real.

“Our baseline assumption is that the mRNA vaccines are functioning similarly, but then you start to see a separation,” said Natalie Dean, a biostatistician at Emory University in Atlanta. “It’s not a huge difference, but at least it’s consistent.”

But the discrepancy is small and the real-world consequences uncertain, because both vaccines are still highly effective at preventing severe illness and hospitalization, she and others cautioned.

“Yes, likely a real difference, probably reflecting what’s in the two vials,” said John Moore, a virologist at Weill Cornell Medicine in New York. “But truly, how much does this difference matter in the real world?”

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