Common side effects of current COVID-19 vaccines include fever, chills, fatigue, and headache, along with pain and swelling around the injection site. The Centers for Disease Control and Prevention explains that these side effects are “normal signs that your body is building protection.”

But what if you do not experience any of these symptoms? Does that mean the vaccine or your immune system is not working? According to the Cleveland Clinic, statistics from the clinical trials of both the Moderna and Pfizer mRNA COVID-19 vaccines show that more than half of the study participants did not have any side effects, but were still 94% protected.

Dr. Thaddeus Stappenbeck, M.D., chairman of the Department of Inflammation and Immunity at the Cleveland Clinic, says that while researchers don’t know why some people experience adverse symptoms and others do not, “the vaccines that have been approved for emergency use are effective whether or not you’ve developed side effects.”

According to Healthline, side effects tend to be more common after the second vaccine does and hit younger people harder because their immune systems are more robust, said Dr. Sharon Nachman, chief of infectious diseases at Stony Brook Children’s Hospital in New York.

“In general, the older the patient is, the less likely the reaction is going to be significant or severe,” she added.

Medical experts note that post-vaccine symptoms and side effects usually clear up within 48 hours after the injection.

Dr. Meg Sullivan, medical director of Mecklenburg Public Health in Charlotte, North Carolina, says that “the lack of side effects does not indicate the vaccine is not working,” according to a local NBC affiliate.

Dr. Stappenbeck, of the Cleveland Clinic, adds people should not worry if they do not experience symptoms.

“You don’t have to feel terrible to prove that you’re protected from COVID-19,” he said.