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Worried about side effects after a COVID-19 shot? Here’s why you probably don’t need to be

Rather than causing any concern, a new study suggests mild post-vaccination symptoms can be signals of vaccine effectiveness, and reinforces the importance of boosters in older people.

Medical consultation. Photo by SHVETS production from Pexels


What is the research about?

The Pfizer-BioNTech and Moderna COVID-19 vaccines have been rolled out to millions of people and have been associated with mild post-vaccine symptoms. However, whether these symptoms are connected in any way to the immune response is largely unknown.

The Framingham Heart Study is an ongoing prospective cohort study assessing risk factors for cardiovascular disease, and these participants also contributed to the Collaborative Cohort of Cohorts for COVID-19 Research (C4R) study, offering a dataset to analyse for links between post-vaccination symptoms and immune response. The paper is published in JAMA Network.

This study could “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.”

What did the researchers do?

The researchers analysed data from 928 adults who self-reported symptoms that they experienced after receiving Pfizer/BioNTech or Moderna COVID-19 vaccinations. The symptoms were common post-vaccine symptoms, which in the vast majority of people only last a few hours or days.

The researchers divided them into systemic symptoms such as fever, chills, muscle pain, nausea, vomiting, headache, and/or moderate to severe fatigue) or local symptoms (injection site pain and/or rash).

The participants also gave a sample of a dried blood to test for antibodies. IgG antibodies to the SARS-CoV-2 spike protein were measured from the sample. Most of the participants were white adults, with an average age of 65.

What did they find?

After a dose of either vaccine, 446 (48%) of participants reported systemic symptoms while 12% reported only local symptoms, and 40% reported no symptoms at all.

Antibody reactivity was observed in 444 (or 99%) participants with either systemic or local symptoms, compared with 98% of those with no symptoms. Although this is not statistically significant, when researchers looked at factors such as time since vaccination or age, people who had systemic symptoms had a marginally greater antibody response than people who had only local or no symptoms.

What does this mean?

The researchers acknowledge a potential inherent bias in their data-gathering as the symptoms are self-reported, which could introduce errors. As the average age was 65, this indicates the finding is important for an older age group, but as participants were mostly white, it’s not easily generalisable.

It’s important to note that if post-vaccine symptoms are severe or do not go away after a few days, people should seek medical help.

Given that most people, whether or not they had side effects, seemed to be sufficiently protected, differences between groups are not that important. People who had no symptoms at all also had strong immune responses, so this does not mean that not having a reaction such as a fever or muscle ache equates to a lesser immune response.

What is important, say the researchers, is that it emphasises that this study could “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.”