Reports of heavy menstrual bleeding or absence of menstruation after mRNA vaccines are being analysed by the European Medicines Agency.
The European Medicines Agency's safety committee says it is reviewing the after women had received COVID-19 vaccines from Pfizer-BioNTech and Moderna, both based on messenger RNA technology.
It was not yet clear whether there was a causal link between the vaccines and the reports, the agency said.
Menstrual disorders can occur due to a range of underlying medical conditions as well as from stress and tiredness, the EMA said, adding that cases of such disorders had also been reported following COVID-19 infection.
Vaccination against COVID-19 was linked with a small, temporary change in menstrual cycle length, according to a recent study funded by the National Institutes of Health which collected data from nearly 4000 users of a smartphone app that tracks menstrual cycles.
But the EMA said in December it had not established a link between changes in menstrual cycles and COVID-19 vaccines after a study in Norway suggested some women had heavier periods after being inoculated.
After reviewing the available evidence, the EMA's Pharmacovigilance Risk Assessment Committee (PRAC) said it decided to request an evaluation of all available data including reports from patients and healthcare professionals, clinical trials and the published literature.
The agency on Friday added that there was also no evidence to suggest that COVID-19 vaccines affected fertility.