Study links Covid-19 vaccination to small increase in menstrual cycle length, but experts say it's no cause for concern

After getting a dose of Covid-19 vaccine, women had an average menstrual cycle length of about one day longer than usual, according to a study published Thursday.

The findings validate some claims from women across social media that the Covid-19 vaccine affected their menstrual cycle. But the change is not clinically significant, and experts say it shouldn't cause worry.

"The bottom line is, we really think these findings are reassuring for health and reproductive health," and they dovetail with similarly reassuring data on the safety of Covid-19 vaccines regarding pregnancy and fertility, Dr. Alison Edelman, an OB/GYN and professor at the Oregon Health & Science University, Portland and the study's lead researcher, told CNN.

But there is an individual relationship with the menstrual cycle that's much more than clinical, Edelman said. And this research provides concrete information to help people understand what to expect, just like any other side effects.

"Let's say nobody told you that you were going to get a fever (after getting the Covid-19 vaccine). It'd be like, 'What just happened?' " she said. "And people have different relationships with their menstrual cycle. For some people, maybe they're planning pregnancy or trying to avoid a pregnancy. Even one day of change -- and that's a mean -- can feel uncomfortable."

The researchers note that the average increase in the length of menstrual cycle "appears to be driven largely" by women who received both doses of an mRNA vaccine -- either Pfizer/BioNTech or Moderna -- within one menstrual cycle. For this group, cycle length increased by an average of about two days. But the change was only temporary and resolved within a couple months, the study found. There was not enough data to say how long the change lasted in the other women.

Regulation of the menstrual cycle can be affected by daily life, environment and health stressors. The researchers ruled out pandemic-related stress as a cause of the changes.

However, they note that mRNA vaccines create a similar robust immune response that could affect the regulation of the menstrual cycle temporarily.

An acute severe illness, such as Covid-19, could be "catastrophic" to this regulation, sometimes permanently, they wrote.

Overall, only about 5% of vaccinated women had a clinically significant change in their cycle of more than eight days, but this rate was about the same among unvaccinated women. Neither vaccinated nor unvaccinated women in the study had a change in the length of their period specifically.

In August, the National Institutes of Health (NIH) announced an investment of about $1.7 million to support five research teams in studying the potential effects of Covid-19 vaccines on menstruation.

This is the first of those studies to publish, an incredibly quick turnaround time for such research.

Dr. Diana Bianchi, director of the NIH's Eunice Kennedy Shriver National Institute of Child Health and Human Development, told CNN that the investment in this research was a direct response to concerns from the American public.

"We were worrying that perhaps the lack of information was contributing to vaccine hesitancy in reproductive age women," said Bianchi, who was not involved directly in the research. There was a desperate need to have scientific evidence to communicate with women about what they can expect when they get vaccinated, and time was of the essence, she said.

A Kaiser Family Foundation survey from December found that nearly 60% of women who are pregnant or trying to become pregnant said they were not confident that the COVID-19 vaccines are safe.

But evidence contradicting those worries continues to accrue.

A study published Tuesday by the US Centers for Disease Control and Prevention found that women who are vaccinated against Covid-19 during pregnancy face no increased risk for preterm or low-weight births.

The CDC recommends vaccination for all women who are pregnant, who were recently pregnant, who are trying to become pregnant or who may become pregnant in the future. Yet vaccine uptake among pregnant women is low: The latest data shows that only about 40% of pregnant women have been vaccinated.

"The study, on the one hand, does validate what some women were saying on social media," Bianchi said. "But overall, at a population health level, this slight change is really not of clinical significance. It should not affect fertility, and the benefits of being vaccinated and not getting Covid -- even a mild version of the Omicron variant -- are so much greater. You really should not be hesitant to be vaccinated."

She also credits women on social media with calling attention to the fact that most clinical trials do not collect data on menstrual cycles, a signal of health and potential fertility that Bianchi says the NIH considers to be like a vital sign.

"Most clinical research trials have not routinely collected information on how an intervention affects the menstrual cycle. To my knowledge, this is the first time that information has been brought up that a vaccine might affect the menstrual cycle. You don't hear this with a flu vaccine, for example," Bianchi said. "So we're hoping that this is really bringing awareness to the importance of collecting information on the menstrual cycle when any intervention is contemplated."

The study researchers analyzed data from about 4,000 women who used the Natural Cycles app to track menstruation, including about 2,400 who became vaccinated over the course of the study and about 1,600 who remained unvaccinated.

This study does not specifically address fertility or other possible changes in menstrual cycles, such as symptoms or unscheduled bleeding. Researchers analyzed data only from women who typically experience regular menstruation cycles, but they note that many do not fit into this category.

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Dr Eleftherios Meridis