We’re more familiar now with the possible side effects from the COVID-19 vaccine interacting with our immune system. Experts stress post-shot issues like fatigue and fever mean the vaccine is working (as long as they aren’t indicative of an allergic reaction).
So, what does this mean for those of us who have no side effects?
We asked vaccine experts to give us the rundown on what side effects mean and whether their severity predicts how effective your immune response will be to the COVID-19 virus.
First, a recap on what causes COVID-19 vaccine side effects.
COVID-19 vaccine side effects are either a physical manifestation of your body’s immune response ― which is the case for most people ― or an allergic reaction, said Jesse Erasmus, acting assistant professor in the department of microbiology at the University of Washington School of Medicine.
Erasmus said the side effects you have from a shot typically depend on the type of vaccine technology that’s used to create the immunization (for example, messenger RNA, or mRNA, is the type of technology the Pfizer-BioNTech and Moderna shots use) and how those components interact with your immune system.
In terms of the coronavirus shots, “all the vaccines that are currently in emergency use authorization have very similar side effect profiles,” said Colleen Kelley, an associate professor of infectious diseases at Emory University School of Medicine and principal investigator for Moderna and Novavax Phase 3 vaccine clinical trials at the Ponce de Leon Center clinical research site in Atlanta.
Kelley thinks the COVID-19 shot side effects mainly stem from the body responding to the spike protein the vaccine introduces to the immune system, which helps it recognize (and then fight off) the spike protein on the coronavirus should it enter the body.
When it comes to allergic reactions to the vaccine, which are rare, a hypothesis for mRNA vaccines is that people may be allergic to a component called polyethylene glycol, a common food additive, Erasmus said.
Why do some people have worse side effects than others?
Based on people’s experiences, it appears that some have worse reactions to the shot than others. But scientifically there aren’t any confirmed reasons for this yet.
“There aren’t really any distinguishing factors that would predispose one individual having more side effects versus the other,” said Richard Dang, a pharmacist and assistant professor of clinical pharmacy at the University of Southern California. “The only thing we’ve seen in the clinical data so far is that younger individuals seem to experience side effects at higher rates than older individuals, and we see that in the real world as well.”
There have been reported cases in which those who previously had the virus endured harsher side effects after they received their vaccines.
“Anecdotally, it does appear that people who may have had COVID-19 before their vaccine do tend to have those longer duration of symptoms,” Kelley added. “But we’re still gathering additional scientific data to really support this.”
Does the severity of side effects have anything to do with how well your body will fight COVID-19 if exposed?
Though it is a valid question, more studies need to be conducted to unpack what the severity of side effects actually mean, said Anna Wald, an infectious diseases physician and researcher in COVID-19 vaccine trials at the University of Washington’s School of Medicine.
But Erasmus, Kelley and Wald all say the effectiveness of the vaccine is unlikely to be determined by how severe your side effects are.
“Remember that most people have mild or no side effects in the clinical trials [for the mRNA vaccines], and yet the vaccine was still found to have 95% effectiveness at protecting them from illness,” Wald noted.
Whether you develop mild or severe side effects, it’s important to know what to do.
Bottom line, the benefits of the vaccines outweigh the side effects. Getting the shot means protecting yourself against severe disease, hospitalization and death from COVID-19.
If you do encounter side effects, there are a few things you can do. At the time of the vaccination, ask the person vaccinating you who best to contact (and how) for follow-up care should you need it, Dang said. You should also wait 15 to 30 minutes at the vaccine site after you receive the shot to make sure you don’t have any severe allergic reactions.
Usually, if you’re experiencing the immune system-related side effects, like fatigue, headache or fever, Kelley said, you can take a pain or fever reducer, such as Tylenol, then take a nap if you’re able. Make sure to stay hydrated and take it easy when you’re feeling off as well.
These issues will likely resolve in one to four days at the most, Kelley said. Anything lasting longer warrants a check-in with your doctor or at the place where you received your vaccine. You should seek emergency care or call 911 if you’re having difficulty breathing or significant swelling.