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If you have no side effects after your COVID-19 vaccine, it doesn’t mean it’s not working. Lea Suzuki/The San Francisco Chronicle via Getty Images
  • Many people who get vaccinated against COVID-19 are experiencing side effects in one form or another.
  • Experts say these indicate your immune system has been activated.
  • But if you don’t have vaccine side effects, you’re still protected.

All data and statistics are based on publicly available data at the time of publication. Some information may be out of date.

Many people who get vaccinated against COVID-19 are experiencing side effects in one form or another — some develop a slight fever, others get a headache, feel nauseous, or are bogged down with fatigue.

Health experts say these reactions are to be expected and show that your immune system is doing its job and learning how to fight the coronavirus.

But those who don’t develop any side effects are left wondering if their immune system is working properly.

There’s been such a focus on the side effects that public health messaging has failed to address the fact that many people will experience no side effects — and that’s okay.

“Even if you don’t feel crummy after your vaccines, chances are your body still had a good, protective immune response,” said Dr. Chris Thompson, an immunologist and associate professor of biology at Loyola University Maryland’s department of biology.

When the vaccines first started to roll out, the public was concerned about side effects — which led medical and public health leaders to help people understand this was the body’s natural response to vaccination, said Brian Castrucci, an epidemiologist and president of the public health nonprofit de Beaumont Foundation.

But the public health messaging often didn’t address what a lack of side effects meant.

“Messaging didn’t address those who didn’t have side effects, leading some to wonder if they were also protected,” Castrucci said.

Even though side effects like muscle aches, fever, or fatigue are signs the immune system is working, a lack of side effects doesn’t mean the shot isn’t working.

In the Pfizer and Moderna clinical trials, a significant portion of participants didn’t experience any side effects, yet they were still highly protected against the coronavirus.

“When you look at the trial data, a little more than half of the participants didn’t have any side effects but they were still more than 90 percent protected after receiving the vaccine,” Castrucci said.

More inclusive messaging would have noted that everyone in the trials, regardless of whether they had side effects or not, achieved more than 90 percent protection from the shots.

“Some people will have side effects, but lots won’t,” Castrucci said. “Either way, just like the trial participants, you can expect to be protected from the virus.”

People’s immune systems react in different ways, with some forming a greater physical response to vaccination.

According to Thompson, people react differently to vaccines due to a handful of contributing factors: health, age, gender, preexisting immunity, genetics, nutrition, environment, and use of anti-inflammatory medicines.

Some research even suggested that the time of day in which a person is vaccinated with the seasonal flu vaccine could affect the immune system’s response.

In addition, people who previously had COVID-19 tend to have stronger reactions to the vaccine.

“If they have immunity to [SARS-CoV-2], they will likely have a stronger reaction to the vaccine,” Thompson said.

There’s also confusion about whether people who are immunocompromised will be protected after vaccination.

People who are immunocompromised still mount an immune response, explains Thompson. They may produce antibodies at a slower rate and they could produce less antibodies overall, but the shot will still likely confer some level of protection.

“This may mean that they do not have as many vaccine side effects, but that does not mean that the vaccine did not work. It probably did!” Thompson said.

Thompson said a person’s reaction to vaccination “really boils down to the innate differences in each person’s biochemical makeup, environment, and personal history.”

Vaccines are designed to have few or no severe reactions, according to Thompson. Still, people often experience side effects after getting vaccinated — and the COVID-19 vaccines are no different.

“Influenza, MMR, Td/DTaP, and shingles vaccines, among many others, all have variable reactogenicity — the ability to cause local or systemic reactions,” Thompson said.

A little inflammation is necessary for the body to develop strong immunity, but we don’t have a way of measuring that level of inflammation and determining how that might reflect a person’s immune response.

It’s tough to pinpoint why some people develop side effects. There’s currently no data suggesting a person’s reaction is any indication of how well their body produces an immune response.

“That’s a question that has yet to be answered,” said Castrucci.

There’s been so much focus on the fact that side effects after vaccination mean the immune system is doing its job, that people who don’t develop any side effects are left wondering if their immune system is working properly.

Health experts say that side effects or no side effects, everyone who gets vaccinated can expect to have protection.

It’s unclear why some people react to vaccines whereas others do not, but it likely comes down to a mix of factors including age, health, environment, nutrition, and gender.