With approximately 19% of the U.S. population fully vaccinated, we have more information than ever about the three COVID-19 vaccines currently available to Americans — and we also have a lot of information about their side effects. Nausea, muscle pain, chills, fatigue, and a fever are all normal reactions. So normal, in fact, that some people might now be more worried about what it means if they feel nothing.
But experts say that both side effects and a lack thereof are no cause for concern. “The reason why people have different levels of side effects or different severity is complex and not fully understood,” Abisola Olulade, MD, a San Diego-based physician, tells Refinery29. However, we do know that side effects are connected to immune response, which is why people who are older — and have weaker immune systems — are actually less likely to experience them. “Essentially, when you get a vaccine, your body recognizes it as foreign, and then it triggers inflammation. And that’s essentially what gives those side effects. So some people may not have that trigger as strongly as others.”
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The way you react to a vaccine depends on a whole slew of factors, including age, preexisting conditions, and genetics. Studies show that people with high levels of estrogen are likely to report stronger side effects, and data released in February indicated that people who have previously had COVID-19 also tend to take longer to shake off the post-vaccine symptoms. Dr. Olulade says that even things like your environment, diet, and stress levels might play a role in your reaction to the shot.
You also might experience a different reaction depending on your vaccine. According to a recent study published in JAMA, about 75% of people said they experienced some sort of reaction after receiving their second dose of the Moderna vaccine, whereas just 64% of people experienced side effects from their second Pfizer-BioNTech shot. The Johnson & Johnson shot, which only requires one dose, has the fewest reported side effects, but it also has the lowest efficacy rate: While the Moderna and Pfizer shots have 94.1% and 95% efficacy rates, respectively, the J&J vaccine is at 72% overall, and 86% against serious disease. (It should be noted that Johnson & Johnson's vaccine is still an important and efficient combat to COVID-19.)
These numbers also prove that side effects don’t dictate vaccine efficacy. In the Pfizer trial, for example, only around half of all test subjects said they had any physical reaction to the vaccine. “They all, regardless of whether or not they had side effects, had really good protection against coronavirus,” says Dr. Olulade. “So that tells you that just because someone doesn’t have side effects, it doesn’t mean the vaccine is not effective.”
But everyone’s immune system is different, and just like it’s perfectly normal not to experience any side effects, it’s also normal if you do feel achy, sick, or exhausted after your vaccine. In fact, it’s just proof that your body is mounting an immune response. “Vaccine side effects are a great thing,” Dr. Olulade adds. “It tells us that your immune system is working and it’s responding the way we want it to, which is very important.” This doesn't mean, though, that if you don't have a side effect, anything is wrong with you.
More good news: Side effects from all three vaccines should go away within a couple days. The only real reason to worry, according to the CDC, is if you’re still experiencing symptoms after a few days, or if the injection site becomes even more tender, red, or swollen after 24 hours. If either of these things happen, call a doctor (or take an antihistamine if you suspect you have a rash or other type of allergic reaction).