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

As millions now know from personal experience, the COVID-19 vaccines can cause temporary side effects. It’s a good sign, as it suggests your immune system is paying attention, preparing you in case of infection with the coronavirus.

But if you don’t experience side effects, does that mean the vaccine did not work?

Good news: The short answer is no, infectious-disease experts say. There is no evidence that a lack of side effects means the vaccinated person is unprotected against COVID-19.

The details require a bit of explanation, but the main reason physicians feel comfortable making that statement is simply math.

In the clinical trials, less than half of vaccine recipients reported moderate or severe episodes of “systemic” side effects such as fever, headache, and fatigue. Yet the drugs prevented most cases of disease, according to those studies. So by the process of elimination, some of that disease prevention must have occurred in the people with mild or no side effects.

We contacted three experts who endorsed that line of reasoning: Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia; Glenn Rall, a Fox Chase Cancer Center immunologist; and Sarah Coles, a family physician and assistant professor at the University of Arizona College of Medicine.


Coles also wrote a blog post about the topic for Dear Pandemic, a group of women physicians and scientists who have been providing plain-language COVID-19 guidance to the public since the spring of 2020.

“The vaccines still work even if you don’t have side effects,” she said. “While many have side effects, many do not.”

Offit said he was unaware of formal studies as to whether the side effects are associated with vaccine effectiveness, but he agreed that a lack of fever or headache is no cause for concern.

“You certainly don’t need to develop side effects to be protected,” he said.


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