Who is likely to have vaccine side effects?

Many people wonder if they can predict whether they will have side effects after getting a dose of COVID vaccine. The short answer is that we can’t predict with certainty.  However, there is some data to guide us.

In general, most people who get either the Pfizer or Moderna vaccine experience some soreness in the arm where they got the vaccine. Many people experience at least one or more other side effects including fatigue, low grade fevers, and muscle aches. These side effects are consistent with what scientists call “reactogenicity”.  

Reactogenicity refers to those side effects that are related to your immune system making antibodies. They can be considered a sign that the vaccine is working. (However, many people successfully make antibodies without any side effects).

The Pfizer and Moderna vaccines require two injections called the “primer” and “booster” doses.  The “primer” or first dose starts the immune response, and the second “booster” dose triggers the body to produce enough antibodies to prevent illness. Because the booster dose (second dose) produces more antibodies, the immune response is stronger. This is why people are more likely to have side effects after the second dose of vaccine.

Similarly, this is why people who have had COVID-19 before may be more likely to have reactogenicity symptoms. The COVID-19 infection they had previously acts as a priming dose and the vaccine doses act like boosters.

Last update: 03 30, 2021, 12:50pm ET

Science review: HAY, ERS