Experiencing side effects like soreness, fever, and fatigue after vaccination may be unsettling, but doctors assure us these are normal signs of a healthy immune response. They indicate your immune system is actively engaging with the vaccine, doing its job to prepare for the real thing.
“If you feel bad after the vaccine, at least feel good about feeling bad,” says Dr. Kevin Dieckhaus, chief of infectious diseases at UConn Health and co-author of on COVID-19 vaccine side effects.
However, the absence of side effects shouldn’t raise alarm bells. A smooth post-vaccine experience doesn’t necessarily mean the vaccine is ineffective.
Why do vaccines cause side effects?
Vaccines work by introducing a weakened or inactive version of a pathogen into your body, triggering the immune system to create antibodies, proteins that fight off infections. This process involves inflammation, which can lead to temporary discomfort like soreness and fatigue.
The intensity of side effects varies depending on the vaccine and individual factors. Even with the same vaccine, two people may experience different side effects due to age, sex, health, and immunization history, among other factors. “The human body is an amazingly varied machine,” Dieckhaus says.
Do stronger side effects mean stronger immune protection?
Some studies have suggested a possible correlation between the severity of side effects and the strength of the immune response. pointed to a similar effect for flu shots.
But have reached that conclusion. “The literature is pretty mixed,” says Florian Krammer, a professor of vaccinology at the Icahn School of Medicine at Mount Sinai. “It’s highly variable.”
Even that have found a correlation have concluded that, when it comes to protection, the difference between people who get side effects and those who don’t is so minimal as to barely matter, Krammer adds.
So is it bad not to get side effects after a shot?
Don’t worry if you feel fine the day after an immunization. While side effects can be a sign of the immune system working, they are not necessary, Krammer says. In fact, vaccine developers usually strive to create a product that works as well as possible while triggering as few side effects as possible.
Almost “everybody gets an immune response” after being vaccinated, Dieckhaus agrees. “If you have symptoms, we just know that your immune response is probably a little bit more robust.” Side effects may be the cherry on top of the sundae, but you still have the sundae no matter what.
In the for Pfizer-BioNTech’s COVID-19 vaccine, for example, less than half of study participants reported side effects, but the shot worked well for the vast majority of people. People who don’t respond well to vaccines typically have a medical reason, such as taking an immune-suppressing drug. “If you’re a somewhat healthy adult and you don’t have side effects, good,” Krammer says.
Plus, trends reported in studies don’t always translate to individual experiences, says Dr. Ethan Dutcher, a postdoctoral scholar at the University of California, San Francisco, and co-author of a recent on the side effects of COVID-19 vaccines. Overall, his team found that people who experienced lots of side effects after their initial COVID-19 vaccines tended to mount stronger antibody responses. But as with any trend, there were plenty of exceptions. “We had a lot of people who didn’t experience tiredness who had higher antibody levels than people who did experience tiredness,” Dutcher says.
And, Dieckhaus says, the immune system is complex. His team’s study, which also found an association between side effects and durability of immune response, measured only one aspect of it: how long it took antibodies to wane after COVID-19 shots. But that’s “just one piece of the puzzle,” he says. There are lots of other variables that influence whether someone gets infected and how sick they’ll be if they do.
The bottom line: make sure you get your vaccines, and don’t worry too much about how you feel afterward. If you have side effects, “you can feel your immune system working,” Krammer says. “But if you don’t feel that, that’s fine too. It’s probably still working.”
