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The Pfizer COVID-19 vaccine is significantly less effective against the delta variant than the Moderna vaccine, according to a new preprint study from the Mayo Clinic that has not yet been peer reviewed.
Both vaccines are still very effective against severe delta variant disease COVID-19, the study authors said. However, Moderna was, on average, found to be 76% effective against delta variant infection compared to Pfizer, which was 42% effective against variant infection.
“The elephant in the room is the amount of mRNA in vaccines,” said Venky Soundarajan, co-founder and chief scientist of Nference, the data analysis arm of the Mayo Clinic based in Cambrdige, Massachusetts.
The Moderna vaccine has a greater amount of mRNA than the Pfizer vaccine, Soundarajan said, which may explain the differences in protection against the delta variant.
Soundarajan also noted that the nanoparticulation and nucleotide sequences used in vaccines are different, but the dominant hypothesis is that Moderna is more effective against the variant due to the greater amount of mRNA in the vaccine.
The Moderna vaccine, especially the second shot, is associated with a greater increase in mild side effects compared to Pfizer, Soundarajan added.
“Come to think of it, we think it might be due to the dose,” he said. “There is more mRNA, which means it stimulates a stronger immune response.”
The study, which is peer-reviewed (a process that typically takes two to three months), involved more than 75,000 people in the Mayo Clinic system. The Mayo Clinic is based in Minnesota.
There are a few “caveats” under consideration, Soundarajan said. On the one hand, since genome sequencing is limited, the study authors estimated the number of suspected delta variant cases using inferred data. In other words, the people studied were not necessarily infected with the delta variant, but were suspected to be because it is the predominant variant in the United States at this time.
The study authors are also uncertain whether the lower effectiveness of the Pfizer vaccine is due to its status as the first vaccine to receive emergency use clearance from the FDA. Because Pfizer’s vaccine was the first approved, it has been rolled out in communities at higher risk for serious side effects from the coronavirus, Soundarajan said. Health workers and older people living in health facilities received the vaccine first, for example, and the data can be skewed as a result.
Soundarajan said the Mayo Clinic is advocating for booster injections, especially using the Moderna vaccine and for those first vaccinated, to boost immunity levels in the community.
“We have to balance this with the vaccination of other countries, like Africa and India,” he said. “Those who are not vaccinated will come back and harm us next year.”
Regarding the Johnson & Johnson vaccine, a recent preprint study found that the single injection vaccine is also likely less effective against the delta variant. Johnson & Johnson previously said their vaccine was effective against the variant even eight months after vaccination.
“The message we wanted to get across was not that people shouldn’t be getting the J. & J. vaccine, but we hope that in the future it will be boosted with another dose of J. & J. or a boost with Pfizer or Moderna, ”Nathaniel Landau, a virologist at NYU’s Grossman School of Medicine, who led the study, told The New York Times.
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