Last month, a real-world study of 1,600 Belgian health care workers found that the Pfizer-BioNTech COVID-19 vaccine produced only half as many antibodies than the similar Moderna shot in patients six to ten weeks after full vaccination. Researchers said the difference might be in part owing to a higher amount of mRNA, the effective ingredient in both vaccines, in the Moderna shot.
Pfizer’s BNT162b2 vaccine contains 30 micrograms of mRNA, while Moderna’s mRNA-1273has 100 micrograms.
In an interview with the Financial Times published on Wednesday, Pfizer’s chief scientific officer Philip Dormitzer said the company chose a lower dose of mRNA in its coronavirus vaccine to minimize the risk of uncomfortable side effects.
“If you look at what’s going on with all the COVID-19 vaccines out there, the derailer has often been adverse events that have cropped up,” he told the newspaper.
Dormitzer said Pfizer used “the minimum dose level” required to induce an immune response that was stronger than contacting COVID-19.
Pfizer’s and Moderna’s COVID-19 vaccines produce similar side effects, including muscle pain, redness and swelling, fatigue, and fever. The CDC has recognized rare side effects, such as a type of heart inflammation called myocarditis, in these mRNA-based vaccines.
The Belgian study, published as a research letter in the Journal of the American Medical Association (JAMA) on August 30, found that antibody levels in people vaccinated with two doses of Moderna shots averaged 2,881 units per milliliter, compared with 1,108 units per milliliter in a similar-sized group who received the Pfizer-BioNTech vaccine.
A similar study conducted in the U.S., published as a letter to JAMA on September 2, found that the Pfizer vaccine generated a lower antibody level in people aged 50 and older than in younger recipients, while the Moderna vaccine saw no difference in antibody levels among age groups. In each age group, Moderna’s antibody levels were consistently higher than Pfizer’s. Researchers, too, said the differences could be explained by the amount of mRNA.
Another important aspect of vaccine effectiveness is how antibody levels change over time. Antibody levels from mRNA-based vaccines have been found to wane after a few months. Researchers in the Belgian study said Pfizer’s and Moderna’s different injection interval might also be responsible for the different antibody levels they observed.
Both vaccines have a two-dose structure. The Moderna shots were administered four weeks apart, compared with three weeks for Pfizer.