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Do COVID mRNA vaccines affect menstrual cycles?

Several scientific studies provide some evidence about whether COVID-19 mRNA vaccines have small and temporary changes to a woman’s menstrual period. Fortunately, the incidence of this effect is rare and does not cause harm to the individual, but women who receive these vaccines must be aware that it may happen.

However, and this is important, COVID-19 itself seems to have a larger impact on menstrual cycles than is observed with the vaccines. Hence, the benefits of the vaccine in preventing hospitalization and death still outweigh this “risk.”

As I usually do, I will examine the results from the papers and provide you with a critique.

doctor writing on a medical chart
Photo by RDNE Stock project on

COVID-19 vaccines and menstrual period articles

In a paper published on 1 January 2024 in Obstetrics and Gynecology, Alexandra Alvergne, PhD, Institute for Evolutionary Sciences, Montpellier University, Montpellier, France, and colleagues 6,514 women from 110 countries representing 32,570 cycles (COVID-19 symptoms: 1,450; COVID-19 vaccination: 4,643; control: 421).

Here are the key results from that paper:

  • The cohort that had COVID-19 experienced a 1.45-day adjusted increase in cycle length during cycle 4 compared with their three previous cycles.
  • The vaccinated group experienced a 1.14-day adjusted increase in cycle length during cycle 4 compared with their pre-event average.
  • The control group (neither vaccine nor disease) experienced a 0.68-day decrease (95% CI -1.18 to -0.19) in a similar period.
  • These changes resolve quickly within the next cycle.

This study showed that the COVID-19 vaccine group did show an increase in cycle length compared to the control group. However, it was 22% shorter than the increase in cycle length experienced by the group that contracted COVID-19. Importantly, the increases in the menstrual cycle reverted to normal during the next month.

Another paper included all women aged 15-50 years who were diagnosed with heavy menstrual bleeding in the hospital between May 12, 2021, and August 31, 2022. Participants were identified in the French National Health Data System, and the study population totaled 4610 women.

Here are the key results:

  • Compared with control women, those hospitalized for heavy menstrual bleeding were more likely to have received their last dose of mRNA vaccine in the previous 1-3 months.
  • The study stated that there was a 20% increased risk of heavy menstrual bleeding in vaccinated women.
  • Importantly, the risk did not appear to be increased beyond 3 months after vaccination.
  • In addition, the risk is very rare. According to this research, the estimated number of cases attributable to vaccination was 8 cases per million vaccinated women.

Another study, from November 2022, concluded that “COVID-19 vaccination may be associated with short-term changes in usual menstrual cycle length, particularly among women whose cycles were short, long, or irregular before vaccination.”

What does this mean?

It appears that COVID-19 mRNA vaccines may have a small and temporary effect on the length of the menstrual cycle and menstrual bleeding. However, and this is important, COVID-19 itself may also cause this. And these changes do not appear to be serious.


Michael Simpson
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