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What is the COVID vaccine doing to women’s periods? We have been hearing reports of the COVID vaccines causing changes in women’s menstrual cycles for almost a year; as of early January 2022, we now have the first set of published data!

Over the course of this pandemic, many women have had real concerns about what the COVID vaccines could do to their health—particularly reproductive health. My own concerns about fertility, knowledge of fertility awareness, PhD in bioengineering, and desire to share truth have led me to help distill the facts on what effects these vaccines have on women’s fertility and pregnancy. I'm excited to see new data specifically studying the vaccines' effects on menstrual cycles.

To put this new data in context, let’s briefly review the history of the COVID vaccines and studies on menstrual cycles.

In 2020 and 2021, the vaccines available in the United States went through clinical trials to determine safety and efficacy. During these trials, researchers specifically asked about known common side effects of vaccines (e.g., “Did you have a fever?”, “Did you have swelling around the injection site?”) and also collected unsolicited reports of side effects. Menstrual changes were not included in the list of specific questions. Additionally, sexually active trial participants were required to commit to using birth control to participate in the trials. As a result, I expect that many changes in natural menstrual cycles were obscured by the large number of people using hormonal contraception. (By the way, excluding menstruating women from research trials is a problem that needs more attention.)

There are thus many possible reasons why not enough people reported menstrual changes during the trials for it to be included in the list of most common side effects:

  • Women didn’t report changes since they weren’t specifically asked about them
  • Women didn’t experience cycle changes because they are not a common side effect
  • Women didn’t experience cycle changes as a result of being on hormonal contraception
  • Cycle changes were too small for most women to notice and report them
  • Too few people were in the trial to detect rare side effects

About 117,00 people total were in the initial trials for the Pfizer, Moderna, and Johnson & Johnson vaccines.

In late 2020 and early 2021, the trials were completed, and the FDA approved the vaccines under emergency use authorization based on the data that they were safe, effective, and that COVID-19 posed a considerable threat. Many people jumped at the chance to be protected against COVID-19—more than 150 million doses had been administered in the United States by May 1, 2021. With such a large increase in sample size, we started to see reports of less common side effects—things that weren’t picked up in the initial studies due to the smaller number of people involved in the clinical trials. For example, this was how we learned that the Johnson&Johnson vaccine rarely causes unusual blood clotting.

Around April 2021, social media ramped up with reports of unusual menstruation following the vaccine: early or later periods, heavier bleeding, heavier cramping, or unscheduled bleeding in women who had stopped menstruating due to hormones or menopause. Dr. Kate Clancy, an anthropology professor at the University of Illinois, launched a survey collecting people’s reports of their menstruation following the vaccine; the survey got a lot of attention on social media. Results from the study have not yet been published.

In August 2021, the NIH announced supplemental grant funding to 5 institutions to study any link between COVID-19 vaccination and menstrual changes. The institutions are Boston University, Harvard Medical School, Johns Hopkins University, Michigan State University, and Oregon Health and Science University.

The first paper from this round of funding was published in early 2022 from Oregon Health and Science University, and as a scientist, I can tell you it’s a beautiful study. Let’s take a look.

What the study on COVID vaccines and menstrual cycles reveals

This study by Edelman et al. focused on changes in cycle length and bleeding length—future studies will need to examine other possible changes. They collected data from 3,959 women who were tracking their cycles for at least six consecutive cycles on the app Natural Cycles. Of those 3,959 women, 2,403 received some form of COVID-19 vaccine, and 1,556 remained unvaccinated during the study. Vaccinated women included in the study tracked at least three consecutive cycles immediately before vaccination and three consecutive cycles immediately after vaccination.

My absolute favorite thing about this study is that they had two excellent controls: (1) comparing vaccinated women to unvaccinated women and (2) comparing post-vaccine cycles to pre-vaccine cycles in the same women. Those two controls really help the scientists understand what changes were caused by the vaccine itself.

In summary, on average, getting a COVID vaccine increased a woman’s menstrual cycle length by less than one day compared to her cycle length pre-vaccine. Vaccination did not change the length of bleeding (menses length).

A small group of women experienced a large “clinically significant” change in cycle length, i.e. eight days or more. The proportion of women experiencing large changes (4.3 – 6.5 percent) was similar in both the unvaccinated and vaccinated groups.

The Moderna and Pfizer vaccines were two-dose regimens at the time of the study, and women who got both doses during a single cycle seemed to have a slightly larger effect: an increase in cycle length of about two days compared to their pre-vaccine cycles. Their cycle lengths returned to normal within two cycles post-vaccination. For women who only got one dose per cycle, the average change in cycle length was even smaller: an increase of 0.24 days for dose one and an increase of 0.58 days for dose two. I have been tracking my cycles for over a decade, and even I would be hard-pressed to notice a change of 0.58 days!

Most vaccinated participants received the Pfizer vaccine (55.2 percent); the authors stated that sub-analyses looking for any effects based on vaccine brand showed no clinical differences in results.

These results confirm women’s self-reports of alterations in cycle length, and make sense based on the interaction between the immune system and the hormonal circuit controlling the menstrual cycle.

To put these changes in context, a separate, very small study (177 women) looked at the effects of COVID-19 infection on menstrual cycles: 20 percent of those patients reported a decrease in menstrual volume and 19 percent reported a longer cycle during infection. These results are not surprising given the well-known effects of stress on the menstrual cycle.

Still, some questions remain unanswered—this Edelman et al. study focused on cycle length, but was unable to assess the vaccines’ effects on menstrual symptoms such as cramping, quantity of blood lost, or unscheduled bleeding.

Additionally, the women included in this study are not representative of all women. To be included, women had to have pre-vaccine cycle lengths of between 24 – 38 days, not be using hormonal contraception, and not be in menopause. The demographics of people using the Natural Cycles app (and thus those selected to be in the study) are more likely to be white (51.5 percent), college-educated (70.2 percent), and have low or normal BMI (45.3 percent) than the overall U.S. population. Seventy-five percent of participants had no children.

I look forward to future studies examining the effects of the vaccine on the remaining populations: those with naturally long or short cycles, people affected with diseases like PCOS, women nearing or in menopause, and more. I will also be curious to learn about any effects getting a vaccine booster may have.

But for now, we now have a well-controlled study saying that yes, COVID vaccines can increase cycle length. We also have data to say that, fortunately, the increase is usually less than one day, and changes in cycle length resolve within two post-vaccination cycles. This data, combined with all the other publications looking at fertility, miscarriage, and stillbirth rates, support the safety of vaccines for women and can assist women in making informed health decisions.

I know that many women concerned about their fertility are hesitant to trust information coming from pharmaceutical companies, which have historically not focused on women's concerns. However, we have a growing body of strong data collected by skilled researchers showing that the COVID vaccines are safe for women and their fertility, and are significantly safer than COVID infection itself. 

And while COVID vaccines may come with side effects for some users—whether soreness, flu symptoms, or adjusted menstrual cycles—the research is finding these effects are minor and short-lived, which unfortunately can’t be said for those affected by the virus.

For continued reading, Meaghan Bond, PhD has distilled more research on the COVID vaccine's effects on women’s fertility and pregnancy in medically reviewed articles at the fertility-awareness education site Natural Womanhood. Look out for her article on the effects of COVID infection on pregnant women and baby loss in the Winter 2021 issue of Human Life Review.