Although vaccines may prevent or lessen the effects of different diseases, many of them produce similar side effects, like fevers and arm soreness. These types of symptoms usually fall under the category of reactogenicity, which refers to expected reactions when the vaccine activates an inflammatory response.
Side effects due to reactogenicity can feel mildly to severely uncomfortable, but are not considered dangerous, and they dissipate within a few days of getting the shot. Reactogenic effects usually mimic common-cold or flu symptoms, such as headaches, muscle aches, fever and chills, because the immune system ramps up in response to the vaccine as if it’s fighting a true infection. Injection site reactions, though not associated with a cold or influenza, are also reactogenic. Other adverse events, like anaphylaxis, and blood clots or autoimmune events (conditions in which the immune system attacks healthy parts of the body), do not fall under the umbrella of reactogenicity.
The Moderna, Pfizer and J&J COVID-19 vaccines are known to be more reactogenic than many vaccines on the market for other diseases, like the measles, mumps and rubella vaccine (MMRV), human papillomavirus (HPV) and the ones for influenza.
“This is higher reactogenicity than is ordinarily seen with most flu vaccines, even the high-dose ones,” Arnold Monto, an epidemiologist at the University of Michigan School of Public Health, told Science in a November 2020 article. Several other experts quoted in the article agreed and emphasized the importance of letting the public know that higher reactogenicity should be expected with Pfizer and Moderna’s vaccines.
Below is a comparison of the prevalence of two of the most common reactogenic side effects: injection site reactions and fevers with different vaccines. In some cases, a range of percentages represented the prevalence of effects. The graph below used the mean of that range. You can find the full ranges in the next graphic “Common Vaccines and Their Side Effects.”
Below is a more detailed breakdown of adverse effects, reactogenic and otherwise, and how frequently they are associated with different vaccines. The graphic does not represent all possible side effects.