FAQ: Has COVID-19 Vaccine Affected Your Menstrual Cycle? New Study Has Answers

A study links Covid-19 vaccination with “an average increase in menstrual cycle length of less than one day.”

2 min read

A new study by the United States’ National Institutes of Health, published in BMJ Medicine, solidifies the anecdotal records of those who menstruate – the COVID-19 vaccination has affected their period flows.

The study links the vaccination with “an average increase in menstrual cycle length of less than one day.”

Here’s what the study found, and what it might mean for people who menstruate.


Who was surveyed?

A total of 19,622 people participated in the study – 14,936 out of these were vaccinated, while the other 4,686 were not. While the study was open globally, a majority of the participants were from the United Kingdom (32 percent), the United States and Canada (29 percent), and Europe (34 percent). 

The vaccinated participants had received doses of either of the following vaccines – Pfizer-BioNTech, Moderna, AstraZeneca, Covishield, Janssen/Johnson & Johnson, Sputnik, Covaxin, Sinopharm, and Sinovac.

How was the study conducted?

Two parallel sets of data were analysed by the researchers for the vaccinated and unvaccinated participants.

The data was from “at least three consecutive cycles before vaccination and at least one cycle after.”

Researchers used data from the fertility tracking app – Natural Cycles.


What did the study find?

The study found that vaccinated participants experienced a 0.71 day increase in their menstrual cycle after the first dose and a 0.56 day increase in their menstrual cycle after the second dose. On average, people who took one dose per cycle saw their menstrual cycles increase by 0.02 days.

However, participants, who took both doses of the vaccine during the same menstrual cycle, saw their cycle increase by 3.91 days. After the vaccination, their cycles increased by 0.85 days.

These changes were irrespective of the vaccine the participants took.

The study also added, “Of the total, 1,342 participants experienced a change in cycle length of eight or more days, comprising 6.2% of vaccinated individuals and 5.0% of unvaccinated individuals. Women who were younger and who had longer cycle length before vaccination were more likely to experience the increase."


What do these findings signify?

According to the study, for menstrual cycles of eight days and less, these variations are normal, and not something to worry about.

But, since menstruation is directly linked to fertility, it could be a cause of concern for menstruators and might result in vaccine hesitancy among people, the study pointed.


What next?

According to the statement released by the NIH, the authors of the study have encouraged more research on the subject, especially related to unexpected vaginal bleeding, menstrual flow, pain, and study on the “physical reasons why such changes might occur.”

Diana Bianchi, director of NIH's Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), said, "These findings provide additional information for counseling women on what to expect after vaccination."

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