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What does it mean if you have no COVID-19 vaccine side effects?

People have reported side effects following vaccination, but not everyone will experience these and experts stress that the vaccine is still working.
/ Source: TODAY

Many have reported experiencing side effects, like fatigue, chills or muscle aches after receiving a COVID-19 vaccination, but others may experience no symptoms or just feel arm pain.

Experts say those symptoms are proof that your immune system is getting ready to fight the virus, but don't worry if you don't have any.

"What we know from the (clinical) trials is that there's not a correlation between not experiencing symptoms and not being able to mount an immune response," said Dr. Albert Ko, a professor of epidemiology and medicine at the Yale School of Public Health in New Haven, Connecticut. "People still mount good antibodies ... whether they experienced symptoms or not after vaccination. People should not suspect that if they don't get symptoms, the vaccine didn't work."

The Pfizer-BioNTech and Moderna vaccines have been shown to have efficacy of around 90%, and the Johnson & Johnson vaccine has a 72% efficacy rate in the United States.

Dr. Anne Liu, an infectious disease physician at Stanford Health Care in Palo Alto, California, said that the "majority of people" in clinical trials for the Moderna and Pfizer-BioNTech vaccines had "local side effects," which means they felt pain or swelling at the injection site.

"As far as more significant, systemic side effects, it was a minority of patients, especially for the first injection," said Liu, referring to symptoms like fatigue, chills, muscle aches or fevers. "Most people are not going to have systemic side effects."

In some rare cases, people have experienced anaphylactic reactions to the vaccine. Some have also experienced mild skin reactions, like delayed redness at the injection site.

Recent research published in the medical journal JAMA, analyzed side effects reported by people participating in the Centers for Disease Control and Prevention's V-safe surveillance system. Participants of V-safe self-enroll and self-report systemic reactions from the day of vaccination through a year after their final dose of the vaccine. Data collected from nearly 2 million participants who received both doses showed that the most frequently reported local and systemic reactions after the first dose was injection site pain and fatigue. Side effects were "substantially greater" after the second dose for both vaccines: over 50% experienced fatigue, 40% had headaches and over 30% had chills. More people who received the Moderna vaccine, compared with the Pfizer-BioNTech vaccine, reported reactions, especially after the second dose.

Dr. David Dowdy, an epidemiologist and associate professor at the Johns Hopkins Bloomberg School of Public Health, said that "every other vaccine we've ever given" has similar effects: Some people may experience "much stronger side effects," which others "don't feel those side effects at all."

"It doesn't mean that the people who have stronger side effects are the ones who are more protected," Dowdy said, comparing it to punching several people on the arm. All will have gone through the experience, but all will react differently: "Some people are going to feel that pain much more strongly than others, but it doesn't mean that those who didn't feel the pain didn't get the effect of that blow. It's much more about how people's bodies are attuned to what their immune system is doing."

"The people who don't have those reactions are still very protected," Dowdy continued.

Liu said that people are more likely to talk about having symptoms, as opposed to people who don't have any.

"Very few people actually go on social media and say 'I got the injections and I felt nothing, it was totally fine,'" Liu explained. "There's definitely a reporting bias and if people do say that, nobody is retweeting it. ... I think people are not necessarily going to broadcast that they were perfectly fine."