Headache, fatigue and muscle pain after Covid-19 vaccination? Is the vaccine the cause of the symptoms - or do they occur for other reasons?

A new Danish study now provides an answer: For the vast majority of the population, there are no signs that the vaccine causes long-lasting symptoms.

The researchers examined three groups based on their attitude toward the Covid-19 vaccine: unconcerned, skeptical and worried individuals. Among the unconcerned and skeptical – who together made up 95 percent of the participants - no more symptoms were found among vaccinated than among unvaccinated individuals.

"For the vast majority of the population, Covid-19 vaccines do not appear to lead to long-lasting non-specific symptoms. However, we cannot completely rule out that a smaller group of people who are worried about the vaccine in advance may experience such symptoms after vaccination," says Christina Bisgaard Jensen, PhD student at the Department of Public Health at Aarhus University, who is behind the study.

Worries can create symptoms

Among the worried – around five percent of the participants – vaccinated individuals more often reported symptoms such as headache, shortness of breath and dizziness compared to unvaccinated individuals in the same group.

According to the researcher, the explanation for the difference may lie in the so-called nocebo effect – the opposite of the placebo effect, where negative expectations about a treatment can trigger symptoms.

"The symptoms are completely real for those who experience them. But that doesn't necessarily mean the vaccine is the cause. When you are worried or stressed, the body can react physically, and it can lead to you feeling more, interpreting signals differently, or actually developing symptoms," says Christina Bisgaard Jensen.

Important knowledge going forward

Although the pandemic is over, vaccination against Covid-19 continues, and uncertainty about side effects still exists among many Danes.

The non-specific symptoms are difficult to investigate in health registers, as they rarely result in an actual diagnosis.

"Our study shows that for the vast majority, Covid-19 vaccines are not associated with long-lasting symptoms. But it also shows that the side effects that some people experience can be connected to worries and expectations. This is important knowledge, because it can help nuance the debate: Not by dismissing that people have symptoms, but by illuminating the many factors that can play a role," says Christina Bisgaard Jensen.

Behind the research result

Study type: Longitudinal cohort study

Collaboration partners: Collaboration between Aarhus University, Aarhus University Hospital, Aalborg University, Aalborg University Hospital and Bispebjerg and Frederiksberg Hospital.

External funding: TrygFonden and the Independent Research Fund Denmark

Conflict of interest: None

Link to scientific article: https://doi-org.ez.statsbiblioteket.dk/10.1093/ije/dyaf173

Contact

PhD student Christina Bisgaard Jensen
Aarhus University, Department of Public Health – Epidemiology
Phone: +45 61 30 29 30
Mail: cbj@ph.au.dk

Associate Professor Dorte Rytter
Aarhus University, Department of Public Health – Epidemiology
Phone: +45 60 38 12 98
Mail: dr@ph.au.dk