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If you felt like absolute garbage the day or two after your last COVID-19 vaccine, there’s a silver lining for you. Feeling those side effects are a sign of a strong immune response, newly published research shows.

The study, published last week in JAMA, looked at self-reported symptoms after vaccination among older American adults. It found people who reported more symptoms after an mRNA dose showed “greater antibody response” when compared to people who either felt nothing or only felt sore at the injection site.

Common side effects from the COVID-19 vaccine and boosters include fatigue, headache, muscle aches, chills and nausea, according to the Centers for Disease Control and Prevention.

Pain, redness and swelling at the injection site are also common – but those don’t correlate with a stronger immune response, according to the new study.

While stronger side effects signal your immune system working, that does not mean a lack of side effects indicates your immune system wasn’t working.

“I don’t want a patient to tell me that, ‘Golly, I didn’t get any reaction, my arm wasn’t sore, I didn’t have fever. The vaccine didn’t work,'” Dr. William Schaffner, an infectious diseases expert and professor at Vanderbilt University Medical Center, told CNN. “This is more to reassure people who have had a reaction that that’s their immune system responding, actually in a rather good way, to the vaccine, even though it has caused them some discomfort.”

Most side effects of the vaccine pass on their own in a few days, says the CDC. Serious side effects, while rare, are possible. Anaphylaxis, or a severe allergic reaction, happens about 5 times per 1 million doses administered, according to the agency.

“For this reason, everyone who receives a COVID-19 vaccine is monitored by their vaccination provider for at least 15 minutes,” the CDC says.