Why do some people feel sick after getting a COVID vaccine and others don’t?

Vaccine Shot

MADISON, Wis. — If you haven’t gotten a COVID vaccine yet, you likely know at least a few people who have. Perhaps you’ve heard stories about how they felt after. Some might get a headache the next day, experience fatigue, or even get a fever.

But experts say you can’t judge the vaccines based on anecdotes from friends or family. If you want to understand the reactions people are having, take a look at the data from each clinical trial.

That data shows younger patients tend to have more systemic reactions than older patients do.

“As you age, the ability of your immune system to recognize and quickly create antibodies and aggressively attack let’s call it an intruder, starts to get not as good,” said Dr. Jeff Pothof, UW Health’s Chief Quality Officer.

Pothof compared a younger person’s immune system to a team of elite special forces. It decreases in strength to more normal forces as you age, but it still does its job.

In Johnson & Johnson’s clinical trial, more than 60% of younger patients had systemic reactions compared to about 45% of patients over age 60.

Pfizer and Moderna’s studies show similar numbers, but not as large of a difference. The amount of reactions increase with the second shot.

“Even within these side effect percentages, when you take that down to the individual person you just can’t know what’s going to happen when you get your vaccine. You might have some symptoms, you might not. They might be mild, they might be a little more irritating for you,” said Pothof.

If you already got your shot and didn’t feel any symptoms, that doesn’t mean your body didn’t do what it was supposed to.

Pothof said fear and anxiety about the vaccines can also create a reaction by making your body release adrenaline. Some might confuse that feeling with a vaccine response.

He said data also shows more women having a reaction to the shot than men.

“We don’t know why, but it might make some sense because it seems like men do worse with COVID. So maybe there’s something about being a man that makes your immune system not work quite as well and something about a woman that makes it a little more aggressive, which is why they fight it off better and then maybe have more increased frequency of side effects of the vaccine,” said Pothof.

He said most reactions are mild. They can be treated with Ibuprofen and Tylenol and typically go away in a couple days. He said the reactions are nothing compared to actually getting sick with COVID.

“It is such a small price to pay to be protected against COVID-19, this disease that turned our world upside down for now more than a year. It just makes it an easy decision,” said Pothof.