Recent data suggests that Moderna's coronavirus vaccine may maintain a higher effectiveness over time than Pfizer's.
Why it matters: The effectiveness gap could always disappear with more data, and both vaccines remain very effective against severe disease. But if the gap does hold up, it raises questions about whether the two vaccines should be treated the same way policy-wise.
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Driving the news: Several studies — both preprints and those that have been peer-reviewed — have found a difference between the two vaccines' effectiveness over time, although some experts have cautioned that this could stem from flawed head-to-head comparisons.
The studies have evaluated different measures of effectiveness, but all have found that effectiveness against severe disease remains relatively high.
"There have been sort of signals from enough separate sources that start to paint a picture that may reflect a real biological phenomenon — a real difference. I’m starting to believe that there’s something underlying it," said Natalie Dean, an Emery professor who specializes in vaccine study design.
Zoom in: In a study released last week, the CDC found that Moderna was significantly more effective against hospitalizations and emergency department or urgent care encounters than the Pfizer or J&J vaccines.
A second smaller study found more similar effectiveness level against hospitalization between the two mRNA vaccines.
Between the lines: Pfizer was the first vaccine authorized for use in the U.S. and began being administered several weeks before the Moderna vaccine.
"Because of the way the rollouts happened, the oldest and most vulnerable and sickest people, like nursing home residents, got Pfizer," said Cornell virologist John Moore.
That means it's possible that some of the effectiveness gap showing up in some studies is a result of Pfizer being administered earlier and in more vulnerable populations.
However, the large CDC study that found a significant difference in the vaccines' effectiveness found that Moderna's was higher across all ages.
Possible reasons for the difference include that Moderna has a much higher dosing regimen than Pfizer, and the second shot is given after a slightly longer interval.
And, while the vaccines are both made using mRNA technology, they have structural differences.
Yes, but: Both vaccines are nearly equal in their capacity to "do what a vaccine needs to do, which is protect against severe illness," said Paul Offit, director of the Vaccine Education Center at the Children's Hospital of Philadelphia.
The bottom line: The Moderna data may more closely resemble Pfizer's after more time passes. But it may be unwise to use one as a proxy for the other.
"It's not clear that any lesson we see from Pfizer will directly translate to Moderna," Dean said. "I think if you asked this question a few months ago, when there really [weren't] any signals of a difference, people would very much lump them together in their mind."
What we're watching: There's a lot more data — particularly from other countries, like Israel — on Pfizer's waning effectiveness, and the effect of booster shots on restoring effectiveness to original levels.
But if that data isn't applicable to Moderna, regulators may not yet have much data to work with when making booster decisions — a process that is already highly controversial.
"We won't know the real Moderna-specific data for some time now, about restoring effectiveness and how durable that is," said Eric Topol, executive vice president of Scripps Research.
Topol said there have been signs Moderna's effectiveness wanes over time to some degree. "It just may be longer and it may be less," Topol added.
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