Study finds slightly longer menstrual cycle after COVID vaccine

COVID-19 vaccines are associated with a temporary average increase in menstrual cycle length of less than 1 day but no change in period length—confirming previous study findings, according to an international study published yesterday in BMJ Medicine.A team led by Oregon Health & Science University researchers analyzed data from 19,622 users of the Natural Cycles fertility-tracking app who had menstrual cycles of 24 to 38 days and were aged 18 to 45 years from Oct 1, 2020, to Nov 7, 2021.

Menstrual cycles are measured from the first day of one period to the first day of the next.Most participants were younger than 35 years and were from the United Kingdom (31.7%), the United States and Canada (28.6%), and Europe (33.6%).Determining prevalence, clear linkA total of 14,936 women were vaccinated and had consecutive data on at least three menstrual cycles before, and one after, the COVID-19 pandemic began, while 4,686 were unvaccinated and had data on at least four consecutive cycles over the same time span.Two thirds (66.5%) of the vaccinated women had received the Pfizer/BioNTech COVID-19 vaccine, 17.5% received Moderna, 9.1% received AstraZeneca/Oxford, 1.9% were given Johnson & Johnson (J&J), and the remainder received the Covishield, Sputnik, Covaxin, Sinopharm, or Sinovac versions.

The Pfizer, Moderna, and J&J vaccines are authorized for use in the United States.The researchers measured the mean change in menstrual cycle length in vaccinated participants by averaging the three cycles before vaccination with the two after the first and second doses of COVID-19 vaccine and then used models to estimate the adjusted change in cycle length between the vaccinated and unvaccinated groups.The authors noted that the US Vaccine