The research provides reassurance to women who might have avoided vaccination because they were concerned about the impact on their periods, the study says.
Researchers from the Universities of Edinburgh, Montpellier, Oxford, Bristol, and Exeter investigated results from a survey conducted in the UK in March 2021.
Participants were asked about any menstrual changes during the pandemic, their Covid-19 vaccination history, and whether they had ever had Covid-19.
The researchers examined results from almost 5,000 pre-menopausal participants who had been vaccinated against Covid-19.
The vast majority – 82 percent – reported no menstrual changes. Only 6.2 percent reported more disruption, 1.6 percent reported less disruption, and 10.2 percent reported ‘other changes’, which could refer to things like changes to cycle length and regularity, or amount of menstrual bleeding.
Of the 18 percent who reported changes, the risk was higher among those who smoked, previously had Covid-19, or who were not using oestradiol-containing contraceptives such as the combined contraceptive pill.
The researchers then looked at a wider group of 12,000 participants, including those who were vaccinated and unvaccinated against Covid-19.
Compared to those who were not vaccinated, and had never had Covid-19, vaccination alone did not show increased abnormal menstrual cycle factors. Those who had a history of Covid-19, however, were at increased risk of reporting heavier bleeding, missed periods, and bleeding between periods.
The study was carried out against a background of public concern that the Covid-19 pandemic had caused disruption to menstrual cycles due to vaccination, infection with the virus, and lifestyle changes.
There has been a lack of research into how much each factor has contributed to the changes and who is most at risk, but this research provides valuable information about the understudied area of periods and Covid-19, experts say.
The team hopes that the results will help healthcare professionals when counseling women about the relative risks of menstrual disturbance when getting vaccinated against Covid-19, compared with having the infection itself, and also help women make decisions about Covid-19 vaccination.
The findings have been published in the journal iScience.