A study published in the BMJ Medicine in August 2022 involving almost 20,000 women found an association between those who received a COVID-19 shot and an increased length in menstrual cycles. Of the 15,000 vaccinated women in the study, two-thirds received Pfizer/BioNTech’s COVID messenger RNA (mRNA) biologic, but the data also included women who received the Johnson & Johnson/Janssen, Moderna/NIAID or AstraZeneca/Oxford University COVID biologics.1
The researchers gathered data internationally from women who were using the menstrual cycle tracking app Natural Cycles between Oct. 1, 2020 and Nov. 7, 2021. In the group of women who were vaccinated, researchers looked at the previous three cycles prior to vaccination to evaluate cycle changes post-vaccine.
The study authors, who were from universities in Oregon, Massachusetts, London, and Edinburgh, concluded:
Compared with the unvaccinated group, vaccinated individuals had an adjusted increase in menstrual cycle length of less than one day with both first and second vaccine doses. Individuals who received two doses of a COVID-19 vaccine in a single cycle had an adjusted increase in cycle length of 3.70 days compared with the unvaccinated.2
The vaccinated cohort had an increased cycle length of 0.71 days following the first vaccine dose. The study also noted a “significant increase” in the rate of participants who had more than an eight-day increase in overall cycle length.
The authors stated in the conclusion:
COVID-19 vaccination is associated with a small and likely to be temporary change in menstrual cycle length.
Prior Studies Found Correlation Between COVID Shot and Menstrual Changes
The findings align with a smaller peer-reviewed study published in the journal Obstetrics & Gynecology earlier this year, which also found that COVID vaccination correlated with a change in cycle length. Women who received two vaccine doses within one menstrual cycle had the most significant delay in their cycles. The National Institutes of Health (NIH) provided $1.6 million to fund the study.3
In July, University of Illinois researchers published a study that found 42 percent of 40,000 participants reported unexpected heavy bleeding following vaccination and 66 percent of postmenopausal women reported breakthrough bleeding.4
Trending conversations about vaccines altering menstrual cycles gained traction largely on social media, where women reported early or absent periods, heavy bleeding, and clotting on various social media platforms.5
Vaccine Clinical Trials Fail to Evaluate Effects on Women’s Reproductive Health
Data from COVID vaccine clinical trials, which was used by the U.S. Food and Drug Administration (FDA) to grant manufacturers Emergency Use Authorization (EUA) to distribute COVID vaccines beginning in December 2020, did not evaluate effects on women’s reproductive health—something not all that uncommon in the research world. Former Pfizer executive Michael Yeadon and feminist author Naomi Wolf voiced their concerns that the vaccines could affect women’s reproductive health.6 Diana Bianchi, MD, director of the NIH Institute of Child Health and Human Development Division stated that safety studies for vaccines do not necessarily consider the reproductive health of women.3
Scientists associated with the Natural Cycles app voiced concerns about any negative publicity regarding fertility and COVID vaccines, which might contribute to “vaccine hesitancy.” They said:
Although small changes in menstrual characteristics might not be meaningful to clinicians and scientists, any perceived effect to a routine bodily function linked to fertility can cause alarm for those experiencing it, and can contribute to vaccine hesitancy.7
Women Were Not Required in NIH Research Until 1990s
Just 29 years ago, federal law began requiring NIH-funded research to include women. In 2015, the American College of Obstetricians and Gynecologists (ACOG) recommended that the menstrual cycle be evaluated as the “fifth vital sign,” yet assessments of impacts on menstrual cycles in clinical studies is still rare. Jason Wright, OB-GYN and editor-in-chief of the Obstetrics & Gynecology journal says that researchers haven’t yet realized the importance of the menstrual cycle to women.6
Heather Huddlestron, MD, an obstetrician-gynecologist specializing in reproductive endocrinology at University of California, San Francisco says that data should have been collected about menstrual cycles during the clinical trials. She stated:
There’s a long history of us not doing a great job at studying women in research studies because of some of this complexity. The train sort of left the station on that because we are now recommending the vaccine for everybody. So it will be very hard for us to generate a control group to answer this question.5
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Click here to view References:
1 Edelman A. et al. Association between menstrual cycle length and covid-19 vaccination: global, retrospective cohort study of prospectively collected data. BMJ Medicine Aug. 8, 2022.
2 Thomas N. Study links Covid-19 vaccination to small, temporary change in menstrual cycle. CNN Health Sept. 27, 2022.
3 Hendler C. NIH-funded study found COVID-19 shots impact menstrual cycles. The Vaccine Reaction Mar. 7, 2022.
4 Cáceres B. Research confirms menstrual cycle bleeding changes after COVID shot. The Vaccine Reaction Jul. 18, 2022.
5 Parpia R. Women report menstrual irregularities after COVID-19 vaccination. The Vaccine Reaction May 10, 2021.
6 Landman K. Why were scientists so slow to study COVID-19 vaccines and menstruation? Vox Feb. 17, 2022.
7 Maguire D. Did your period change after getting your COVID-19 vaccine? A new study suggests you weren’t imagining it. ABC News Sept. 30, 2022.