A U.S. cohort study assessing the effect of COVID-19 shots on menstrual cycles was peer reviewed and published in the journal Obstetrics & Gynecology on Jan. 5, 20221 after many women spoke out about changes to their menstrual cycles after receiving COVID shots. The U.S. National Institutes of Health (NIH) provided $1.6 million to fund the research conducted at five universities.2
Researchers looked at 3,059 women’s menstrual cycles that were recorded in an app called Natural Cycles three months prior to vaccination and three months post first dose. The results were compared to six cycles of unvaccinated women. 55 percent of the 2,403 vaccinated women in the study received Pfizer/BioNTech’s BNT162b2 (also known as “Comirnaty”) messenger RNA (mRNA) COVID biologic, 35 percent received Moderna/NIAID’s mRNA mRNA-1273 (“Spikevax”) biologic, and seven percent received Johnson & Johnson/Janssen’s human adenovirus vectored Ad26.COV2.S vaccine.3
Minor Changes in Cycle Length After COVID Vaccinations
Researchers found that vaccination was associated with a minor, less than one day, change in cycle length for the vaccinated women, and no change in menses length.4 The study conclusion section begins by stating that concerns about the COVID shots causing abnormal menstrual changes vaccination may lead to vaccine hesitancy. It appears the study was conducted to reassure women the vaccine would only cause minor changes to their menstrual cycle.5
The women who had the greatest delay in their cycles were the ones who received two vaccine doses within one menstrual cycle. They averaged a two-day unadjusted increase in cycle length with 10.6 percent of the women having an increase of eight days or more in the length of their cycle compared to 4.3 percent in the unvaccinated group.6
Kim Makay, MD, a gynecologist at SSM Health, said that the changes to menstrual cycles should only last approximately three to four months. She added:
The vaccine does impact women’s menstrual cycles. The vaccine has an impact on the body, especially when it’s first given; that impact can be stressful, that stress can impact the internal clock a woman has.7
No Human Reproductive Health Studies Conducted Before FDA Gave EUA to COVID Vaccine Manufacturers
Researchers suggest that the COVID shots generate a robust immune response which affects the hypothalamic-pituitary-ovarian axis and influences the timing of menstrual cycles. Researchers admit that this study is limited by the pool of woman using Natural Cycles app, who tend to be white, college educated and have lower body mass indexes (BMIs) than the rest of the population. The study is further limited by only looking at women with normal menstrual cycles and excluding women if they have thyroid problems, polycystic ovarian syndrome, endometriosis or used emergency contraception during the study.8
One of the lead researchers, Alison Edelman, MD, MPH, said:
On a personal level, any noticeable change to a person’s cycle–whether it be related to vaccination or other environmental stressors–can indeed feel significant. While reassuring, the study findings may also be validating to individuals who experienced menstrual disruption following vaccination.9
Prior to the U.S. Food and Drug Administration (FDA) issuing the emergency use authorization (EUA) to COVID vaccine manufacturers, no human studies were conducted to determine the effects of the COVID shots on menstruation and reproductive health. Changes to menstruation is not listed as a potential side effect of the vaccine.10
Diana Bianchi, MD, director of NIH’s Institute of Child Health and Human Development Division pointed out there is a lack of studies investigating the effect of the COVID vaccine on menstruation and this indicates that “safety studies for vaccines … are not necessarily thinking about the reproductive health of women.” She added:
We hope that one of the things that’s going to come out of this is that questions will be added to clinical trial studies to include any changes in menstrual health.11
Researchers at US Universities and in European Union Conducting Research on Menstrual Disorders After COVID Shots
Approximately 4,000 women reported changes in their menstrual cycle to the Vaccine Adverse Events Reporting System (VAERS) in the United States and approximately 38,000 women reported changes in their cycles to a similar database in the United Kingdom prior to this study being conducted.12
Further research on the effect of COVID shots on menstrual cycles is currently being conducted at Boston University, Harvard Medical School, Johns Hopkins University and Michigan State University.13 The European Medicines Agency (EMA) of the European Union (E.U.) is also investigating menstrual disorders that develop after COVID-19 vaccinations.14
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1 Edelmen A, Boniface E, Benhar E et al. Association Between Menstrual Cycle Length and Coronavirus Disease 2019 (COVID-19) Vaccination. Obstetrics & Gynecology Jan. 5, 2022; 10.1097.
2 Kindelan K. COVID-19 vaccine linked to small, temporary changes in menstrual cycles, study finds. ABC News Jan. 7, 2022.
3 Edelmen A, Boniface E, Benhar E et al. Association Between Menstrual Cycle Length and Coronavirus Disease 2019 (COVID-19) Vaccination. Obstetrics & Gynecology Jan. 5, 2022; 10.1097.
7 Molesky C. New temporary side effects linked to COVID-19 vaccine. NBC 15 Feb. 7, 2022.
8 Edelmen A, Boniface E, Benhar E et al. Association Between Menstrual Cycle Length and Coronavirus Disease 2019 (COVID-19) Vaccination. Obstetrics & Gynecology Jan. 5, 2022; 10.1097.
9 Kindelan K. COVID-19 vaccine linked to small, temporary changes in menstrual cycles, study finds. ABC News Jan. 7, 2022.
10 Parpia R. Women Report Menstrual Irregularities After COVID-19 Vaccination. The Vaccine Reaction May 10, 2021.
11 Mcshane J. Women said the COVID vaccine affected their periods. Now more than $1.6 million will go into researching it. The Seattle Times Sept. 7, 2021.
12 Cao S. How Do COVID-19 Vaccines Affect Menstrual Cycles? New Studies Provide Answers. Observer Feb. 5, 2022.
13 Kindelan K. COVID-19 vaccine linked to small, temporary changes in menstrual cycles, study finds. ABC News Jan. 7, 2022.
14 Reuters. EU investigates reports of menstrual disorders after mRNA COVID shots. Feb. 11, 2022.