A vial of COMIRNATY (Pfizer/BioNTech) and a vial of Moderna COVID-19 vaccines.
Enlarge / A vial of COMIRNATY (Pfizer/BioNTech) and a vial of Moderna COVID-19 vaccines.

The vaccine made by Pfizer/BioNTech and Moderna has proven to be very effective in preventing the spread of the coronaviruses. Despite their similar design and efficacy, the two vaccines are not exactly the same and our immune systems do not respond to them in the same way.

There was some data that showed startling differences in the effectiveness of the two vaccines, even though both shots performed nearly identically in Phase III clinical trials. Pfizer's effectiveness against infections fell to 42 percent, while Moderna only fell to 76 percent, according to a study.

There is evidence that the two vaccines spur the immune system to produce slightly different anti-SRS-CoV-2 antibodies.

The vaccines can bind to the virus and prevent it from infecting cells. The vaccines generated different antibody profiles according to the study. The Pfizer/BioNTech vaccine had a skewed response to a class of antibodies called IgM, which are found in the blood. The Moderna vaccine generated elevated levels of IgA antibodies, a class of antibodies found on the respiratory tract, where the first infections of the coronaviruses occur. The Moderna vaccine stimulated higher levels of natural killer cells. It generated higher levels of immune cells that fight invading germs.


Detailed differences

The Moderna vaccine and the Pfizer/BioNTech vaccine had differences in the number of people who had their antibody profiles compared. The participants were mostly healthy young female medical workers, which is not representative of the population as a whole. The study did not look at immune responses over time. After each participant received a second vaccine dose, the researchers looked at their antibody profiles.

Alter and her colleagues wrote that the data provided evidence for potential nuanced differences in the quality of the humoral immune response. Though both vaccines produce strong immune responses overall, these slight antibody differences might provide insights into potential differences in protective immunity conferred by these vaccines.

Alter and her colleagues will have to do more research to find out if the differences are related to vaccine effectiveness. They will need to do more research to understand what is causing the differences. The Pfizer/BioNTech and Moderna vaccines are given at different times and at different doses. The Pfizer/BioNTech vaccine is given as two 30 microgram doses three weeks apart.

The immune system responds to the vaccines in different ways. Researchers could use those differences to create a vaccine that gives the strongest protection. The findings make a case for people to mix-and- match vaccine boosters if they have started with Pfizer/BioNTech vaccine. Changing to a different vaccine for a booster could provide more protection.