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 yet to be peer-reviewed.
Both vaccines are still highly effective against severe disease from the delta variant COVID-19, the study authors stressed. However, Moderna was found to be, on average, 76% effective against infection from the delta variant compared to Pfizer, which was 42% effective against infection from the variant.
“The elephant in the room is the amount of mRNA in the vaccines,” said Venky Soundarajan, the cofounder and chief scientist at Nference, the data analytics 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 account for the differences in protection against the delta variant.
Soundarajan also noted that the nanoparticulation and nucleotide sequences used in the vaccines are different, but the prevailing 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.
“Thinking back, we think that might be because of the dose,” he said. “There is more mRNA, which means it boosts a stronger immune response.”
The study, which is undergoing peer review (a process that typically takes two to three months), looked at over 75,000 people in the Mayo Clinic system. The Mayo Clinic is based in Minnesota.
There are some “caveats” to the study, Soundarajan said. For one, genome sequencing is limited, so the study authors estimated the number of suspected delta variant cases using inferred data. In other words, those studied were not necessarily infected with the delta variant, but were suspected as being so as it is the prevailing variant in the U.S. right now.
The study authors are also unsure whether the lower effectiveness of the Pfizer vaccine is due to its status as the first vaccine to receive FDA emergency-use authorization. Because Pfizer’s vaccine was the first approved, it was rolled out to communities with higher risks of contracting serious side effects from the coronavirus, Soundarajan said. Health care workers and elderly people living in care facilities received the vaccine first, for example, and the data may be inflected by this fact.
Soundarajan said the Mayo Clinic is advocating for booster shots, especially using the Moderna vaccine and for early vaccine recipients, to boost levels of immunity in the community.
“We need to balance that with getting other countries, like Africa and India, vaccinated,” he said. “Them not being vaccinated is going to come back and hurt us next year.”
As for the Johnson & Johnson vaccine, a recent preprint study found that the one-shot vaccine is also likely less effective against the delta variant. Johnson & Johnson previously said its vaccine is effective against the variant, even eight months after vaccination.
“The message that we wanted to give was not that people shouldn’t get the J.&J. vaccine, but we hope that in the future, it will be boosted with either another dose of J.&J. or a boost with Pfizer or Moderna,” Nathaniel Landau, a virologist at N.Y.U.’s Grossman School of Medicine, who led the study, told the New York Times.