New study reveals side effects of COVID-19 vaccination

Denmark has been at the forefront of COVID-19 vaccine research, and Aarhus University recently published two groundbreaking studies on the side effects of the COVID-19 vaccine. These studies, based on the Danish BiCoVac cohort, shed light on common side effects experienced by people after vaccination, as well as the impact on the menstrual cycle in women.

Study of acute side effects

The first study looked at the acute side effects of the COVID-19 vaccination. Researchers found that the most common side effect after the first injection was redness and pain at the vaccine injection site, which occurred in 20% of patients. After the second and third doses of the vaccine, fatigue was the most common side effect reported, at 22% and 14%, respectively. Other common side effects included malaise, fever and general discomfort.

Facial paralysis and allergic reactions were reported as rare among those vaccinated in Denmark. Researcher Kristoffer Torp Hansen, the study’s first author, noted that women, those aged 25-35 and those who had previously had COVID-19 were more likely to report side effects.

Differences in side effects by vaccine

The study also found that the type of vaccine administered played a role in the occurrence of side effects. People who received the AstraZeneca vaccine reported more side effects after the first dose compared to those who received other vaccines. On the other hand, those who received Moderna vaccine reported more side effects after the second and third doses compared to those who received Pfizer-BioNTech vaccine.

Menstrual cycle changes and COVID-19 vaccination

The second study focused on menstrual cycle changes in women after COVID-19 vaccination. The researchers found that 30% of menstruating women reported changes in their menstrual cycle after vaccination. Factors such as stress, age, and smoking were found to correlate with these changes. In addition, menstrual cycle changes were more likely to be reported by women who had concerns about the vaccine, had previously had severe COVID-19 infections, or had multiple reactions to the vaccine.

Expert opinions and citations

Dr. Kristoffer Torp Hansen, a researcher in the Department of Public Health, stressed the importance of understanding the demographic most susceptible to side effects: “Women, people aged 25-35 years, and those who had taken Covid-19 prior to vaccination reported experiencing side effects more frequently than men, older adults, and those who had not previously taken Covid-19.”

Christina Bisgaard Jensen, PhD, first author of the study of menstrual cycle changes, highlighted factors contributing to these changes: “We also found that women who were concerned about the Covid-19 vaccine, had a severe Covid-19 infection, or reported multiple reactions to the vaccine were more likely to report menstrual cycle changes after vaccination.”


The study from Aarhus University provides valuable information on the side effects of vaccination with the COVID-19 vaccine in Denmark. While mild side effects such as redness, pain and fatigue are common, severe adverse reactions are rare. Research also sheds light on the effects of vaccination on the menstrual cycle in women. Understanding these side effects and their associations can help health care providers better inform and support people throughout the vaccination journey.

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