Cohort profile: The BiCoVac cohort - a nationwide Danish cohort to assess short and long-term symptoms following COVID-19 vaccination.

BiCoVac is a population-based Danish cohort aiming to examine whether Coronavirus disease 2019 (COVID-19) vaccines are associated with non-specific symptoms beyond the specific protection of COVID-19. Data were collected by four questionnaire surveys between May 2021 and July 2022 and the questionnaire distribution was aligned with the Danish COVID-19 vaccination program. All surveys collected self-reported information on symptoms (e.g., headache, nausea, and fatigue). The baseline survey additionally gathered information on lifestyle and health. Survey data were combined with data from the Danish registers including information on COVID-19 vaccination and COVID-19 test results. A total of 911,613 (25% of all Danish citizens aged 16 to 65) were randomly sampled for the cohort and 252,401 initiated the baseline questionnaire. Of these, 59% (n = 149,070) participated in the 1st follow-up, 43% (n = 107,655) in the 2nd follow-up, and 25% (n = 63,737) in the 3rd follow-up. Women and individuals above 40 years of age were more likely to participate. Among vaccinated respondents, 25-38% reported moderate to severe immediate symptoms following COVID-19 vaccination, varying by vaccine doses. Females, younger individuals, and those with prior COVID-19 reported more immediate symptoms. Results of potential non-specific symptoms following COVID-19 vaccination did not reveal higher risk of involuntary movements among vaccinated individuals compared to unvaccinated individuals. Currently (December 2024), we are further investigating the effects of COVID-19 vaccines on other non-specific symptoms and exploring whether specific characteristics render some individuals more susceptible to report non-specific symptoms. In addition, long-term symptoms following COVID-19 are being investigated.