Health Advice



Side effects mean your COVID-19 vaccine is working. But what if you don’t have a reaction?

Tom Avril, The Philadelphia Inquirer on

Published in Health & Fitness

To put it another way, both phenomena — acquiring immunity and experiencing side effects — are set in motion by the same series of inflammatory warning signals in the immune system once you get injected.

But just because this initial inflammation is successful in triggering the production of antibodies and other defenses against disease, it does not necessarily cause a fever in the bargain. There is not a strong correlation between the two things, said Coles, who chairs a commission on public health and science for the American Academy of Family Physicians.

And most other modern vaccines cause few, if any, side effects, yet they work well, said Rall, coauthor of Principles of Virology, a standard text in the field.

“We normally don’t see appreciable side effects following vaccination,” he said. (Among the exceptions, besides the vaccines for COVID-19, are those that protect against shingles.)

Whether people experience side effects might be determined by other factors besides their immune systems, including fatigue, stress, and how they perceive pain, Coles said.

The rates of various side effects are slightly different in each of the COVID-19 vaccines, and in those that require two doses, they tend to be more common after the second dose.


With the vaccine made by Pfizer Inc. and BioNTech SE, for example, 3.7% of trial participants aged 18 to 55 reported a fever after the first dose, and 15.8% reported a fever after the second dose — usually no higher than 102 degrees. Half of recipients reported a headache after the second dose, but most were moderate or mild.

All of these impacts are transient, generally lasting a day or two. And in rare cases of a serious allergic reaction called anaphylaxis within minutes after injection, physicians can readily treat that with an EpiPen or similar device containing epinephrine.

A final note: Offit, Rall, and Coles were talking about systemic side effects, not “local” effects such as arm pain or redness at the injection site. Those inconveniences do seem to strike most recipients of the vaccines.

But given the alternative — a case of COVID-19 — it’s worth it.

©2021 The Philadelphia Inquirer, LLC. Visit at Distributed by Tribune Content Agency, LLC.