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.
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.
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.
Several factors might underlie the divergence. The vaccines differ in their dosing and in the time between the first and second doses.
In the midst of a pandemic last year, the companies had to guess at the optimal dose. Pfizer went with 30 micrograms, Moderna with 100.
Moderna’s vaccine relies on a lipid nanoparticle that can deliver the larger dose. And the first and second shots of that vaccine are staggered by four weeks, compared with three for the Pfizer-BioNTech vaccine.
Ultimately, both vaccines are still holding steady against severe illness and hospitalization, especially in people under 65.
Scientists had initially hoped that the vaccines would have an efficacy of 50 or 60 percent. “We would have all seen that as great result and been happy with it,” John Moore, a virologist at Weill Cornell Medicine in New York said. “Fast forward to now, and we’re debating whether 96.3 percent vaccine efficacy for Moderna versus 88.8 percent for Pfizer is a big deal.”
