The study tested doses of each vaccine ranging from 10 micrograms to 100 micrograms. The 30 microgram dose of B2 is in clinical trials.
With the original vaccine, called BNT162b1, or B1 for short, patients between the ages of 18 and 55 had side effects suspected of being related to the vaccine 50% of the time at the 30 microgram dose. People aged 65 to 85 had related adverse events 16.7% of the time.
For the second vaccine, BNT162b2, or B2, patients aged 18 to 55 years experienced adverse reactions suspected to be vaccine-related 16.7% of the time, and no suspected vaccine-related adverse reactions were reported in patients aged 65 and 85.
Both vaccines use mRNA – the genetic messenger the body uses to turn the DNA code into proteins – packaged in a fatty capsule, called a lipid nanoparticle, which allows it to enter cells. MRNA instructs cells to make a protein, which then triggers the immune system to work. For the B1 vaccine, the mRNA encoded the part of a SARS-CoV-2 virus protein that binds to a receptor on human cells in order to enter them. The B2 vaccine makes all of this protein, known as the spike protein.
Using the complete spike protein may allow the immune system to find other ways to detect and attack the virus. Chemical changes in mRNA may also explain part of the difference. Although the same dosage, by weight, was given to patients with each vaccine, the B2 vaccine would have fewer particles because the full length mRNA is heavier.
The side effects monitored were mainly those one would expect from a vaccine injection, including pain at the injection site, fever, chills, headache, and muscle or joint pain. No elderly person who received B2 reported redness or swelling at the injection site.
The average level of antibodies to the virus in older adults was only 41% of that seen in younger participants. However, it was still higher than the level of antibodies seen in recovered patients, the authors said.
All of the patients in Study B2 were white and non-Hispanic, with more older women than older men participating. Younger patients had a median of 37 years, while the oldest had a median of 69 years.
Pfizer said some data from its large B2 vaccine study may be available as early as October.