Some people are hesitant to get vaccinated because they fear vaccine side effects, such as pain, fever, and fatigue. But if you ask your doctor, they’ll tell you that there is a silver lining to these unpleasant symptoms. These symptoms are signals that the immune system is activating in response to the injection and doing its job.
“Even if you feel sick after getting the vaccine, at least make yourself feel good about feeling sick,” said David E., chief of infectious diseases at UConn Health, who is working on the COVID-19 vaccine. says Dr. Kevin Diekhaus, co-author of a 2023 study on the side effects of .
But is it a bad sign that there are no side effects after the injection? If the pain is gone the next day, does that mean the vaccine didn’t work?
Probably not. Here’s why:
Why do vaccines cause side effects?
Vaccines work by mimicking an infectious disease. These introduce weakened or destroyed forms of specific pathogens into your body, allowing you to practice defending yourself against real pathogens. This involves creating antibodies, which are proteins that attack foreign invaders.
When your immune system activates in response to the injection, it can cause inflammation and make you feel sick the next day. Getting the flu shot doesn’t mean you’ll get the flu, and you won’t actually be infected, but you may temporarily experience the same symptoms.
However, side effects vary. Some shots produce more results than others. And even two people given the exact same vaccine can react very differently for a variety of reasons, including age, gender, health, vaccination history, and other factors that scientists don’t fully understand. There is a gender. “The human body is an incredibly versatile machine,” says Diekhaus.
Does stronger side effects mean stronger immune defenses?
Studies on COVID-19 vaccinations show at least a moderate association between the number and severity of side effects people experience after vaccination and the strength and durability of their immune response. It turned out that there is. A 2022 study points out that the flu shot has a similar effect.
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However, not all studies reach such conclusions. “The literature is quite complicated,” says Florian Kramer, a professor of vaccinology at the Icahn School of Medicine at Mount Sinai. “It’s very variable.”
Even the few studies that have found a correlation have concluded that when it comes to protection, the difference between those who receive side effects and those who don’t is so small that it hardly matters, Cramer added.
So, is it bad that there are no side effects after the injection?
There is no need to worry if you feel well the day after vaccination. Cramer says side effects can be a sign that the immune system is working, but they aren’t necessary. In fact, vaccine developers typically strive to create products that work as effectively as possible while minimizing side effects.
Diekhaus agrees that almost “everyone gets an immune response” after getting vaccinated. “If you have symptoms, you know your immune response is probably a little bit stronger.” Side effects may be the cherry on top of the sundae, but there’s a sundae no matter what.
For example, in initial clinical trials of Pfizer-BioNTech’s COVID-19 vaccine, less than half of study participants reported side effects, but the vaccine was effective in the majority of people. People who don’t respond well to vaccines usually have medical reasons, such as taking immunosuppressant drugs. “If you’re a reasonably healthy adult and don’t have any side effects, that’s fine,” Cramer says.
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Additionally, trends reported in studies do not necessarily translate to individual experience, says Ethan, a postdoctoral fellow at the University of California, San Francisco and co-author of a recent study on side effects of COVID-19 vaccines. Dr. Dutcher says: . Overall, his team found that people who experienced more side effects after their first COVID-19 vaccination tended to develop stronger antibody responses. But like any trend, there were plenty of exceptions. “There were many people who didn’t feel fatigued who had higher antibody levels than people who were fatigued,” Dutcher says.
And Diekhaus says the immune system is complex. His team’s research also found a link between side effects and the durability of the immune response, but it only measured one aspect: how long does it take for antibodies to wane after a COVID-19 vaccination? The question was, how long did it take? But that’s “just one piece of the puzzle,” he says. There are many other variables that affect whether someone gets infected and, if they do, the severity.
In conclusion, be sure to get the vaccine and don’t worry too much about your health afterward. Even when there are side effects, “you can feel your immune system working,” Cramer says. “But if you don’t feel that way, that’s okay too. It’s probably still working.”
