Washington: COVID-19 vaccination is associated with an average increase in menstrual cycle length of less than one day, a study published in the British Medical Journal has found.
The researchers from Oregon Health & Science University, US, determined that on average, vaccinated people experienced an increase of less than one day in each menstrual cycle in which they were vaccinated.
They found a 0.71 day increase after the first vaccine dose and a 0.56 day increase after the second shot. The study found that women who received both doses in a single cycle experienced a 3.91 day increase in cycle length.
After vaccination, cycle length had increased by only 0.02 days for individuals who received one dose per cycle, and by 0.85 days for those who received both doses in one cycle, compared to participants who were not vaccinated.
Changes in cycle length did not differ according to the type of vaccine received, the researchers noted.
A change in cycle length of less than eight days is considered within the normal range of variation, they said.
Although small menstrual changes may not be meaningful to health care professionals and researchers, the study authors noted that perceived changes in a bodily function linked to fertility may be alarming to those experiencing it and could contribute to vaccine hesitancy.
A total of 19,622 individuals participated in the large, international study. Of these, 14,936 were vaccinated and 4,686 were not.
The researchers analysed data on at least three consecutive cycles before vaccination and at least one cycle after.
Data from at least four consecutive cycles were analysed over a similar time interval for unvaccinated participants.
Of the total, 1,342 participants experienced a change in cycle length of eight or more days, comprising 6.2 per cent of vaccinated individuals and 5 per cent of unvaccinated individuals.
Women who were younger and who had a longer cycle length before vaccination were more likely to experience the increase. The researchers analysed de-identified data from the fertility tracking app, Natural Cycles.
Users of the app provided it information on their temperature and their menstrual cycles.
Worldwide rollout of COVID-19 vaccines allowed the study authors to expand on their original study of people in the US.
The study included data from participants throughout the world, but most were from the UK (32 per cent), the US and Canada (29 per cent) and Europe (34 per cent).
In addition to the messenger RNA COVID-19 vaccines (Pfizer-BioNTech and Moderna), participants received vaccines made from engineered viruses (AstraZeneca, Covishield, Johnson & Johnson and Sputnik), and inactivated viruses (Covaxin, Sinopharm, and Sinovac).
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