Is your COVID vaccine working if you don’t experience side effects? Experts weigh in

Side effects such as fatigue, headache and muscle aches after receiving a COVID-19 vaccine are normal and should be expected, but they are not guaranteed. While some people are delighted to have a pain-free vaccination experience, others may worry their lack of reactions is a sign the shot isn’t working.

But don’t worry if you miss out on the joy that is cold sweats and achy bones, experts say. No immune system is like the other, so some people may not experience side effects at all, even though their bodies are working just as hard.

“Although having symptoms is a sign that the immune system is reacting to the vaccine, the absence of symptoms does not mean that the vaccine is not working,” California’s San Bernardino County Health Officer Dr. Michael Sequeira said, according to The Mercury News. “Follow-up studies with antibody measures and epidemiology studies have shown that the overwhelming majority of those who experience no symptoms after the vaccine have developed immunity.”

Clinical trials for the two-dose Pfizer and Moderna COVID-19 vaccines showed that just over half of the participants who received the shots experienced side effects, with reactions a bit more common for the Moderna shot. That means about 50% of people never showed symptoms, aside from temporary injection site pain, and still benefited from about 95% protection against symptomatic COVID-19 — and virtually 100% protection against severe disease and death.

Side effects following the single-dose Johnson & Johnson coronavirus vaccine were even less common, despite an overall efficacy rate of about 72% in the U.S. and full protection from severe disease and death.

Generally, younger people and women tend to experience more or worse side effects after vaccination than their older and male counterparts, studies have shown. Experts speculate it’s because younger people and women have more robust immune systems that react more strongly to the vaccines.

“Unfortunately, there is not an easy way to tell if the vaccine worked because ... not everyone develops side effects and the vaccine has been shown to be effective even in people who had no side effects at all,” Dr. Kelly Elterman, an anesthesiologist in San Antonio, Texas, wrote in GoodRx. “The good news is that because the vaccines are highly effective, it’s very likely that they work even if you don’t have any reaction to them.”

And for many who don’t feel side effects after the first dose of a two-dose vaccine, the second shot might yield some reactions, or make them more intense than the first round of vaccination.

The first shot of the Pfizer or Moderna vaccine floods your immune system with instructions that teach your cells to pump out harmless proteins similar to those the coronavirus uses to infect people. Meanwhile, special cells in your immune system recognize the proteins as foreign invaders and send signals to other immune cells to fight them off.

The end result is an army of antibodies that are primed to spot and kill real coronavirus proteins if your body ever encounters them.

The second dose, which repeats this process, is a firm reminder of the threat of infection. Not only does the threat make the immune system work even harder to nail the fight down, but it also solidifies maximum protection against COVID-19, a feat one dose can’t do alone.

People who have had COVID-19 are also more likely to experience side effects after vaccination than those who have not.

A study of more than 280,000 vaccinated people found that individuals with a history of coronavirus infection were almost two times as likely to feel one or more side effects after getting the Pfizer vaccine.

“A stronger response may indicate evidence of an increased immune response,” a statement said.