Last week, University of Illinois researchers published a study funded by the National Institutes of Health (NIH) in the journal Science Advances on menstrual cycle bleeding changes after COVID shots. The study featured a web-based survey of almost 40,000 individuals and found 42 percent of those who were menstruating reported unexpected heavy bleeding after receiving a COVID shot, while 66 percent of postmenopausal individuals reported breakthrough bleeding.1
The study included respondents aged 18 to 80 who were considered to be “fully” vaccinated. It did not include a representative unvaccinated group for comparison.
History of Menstrual Difficulties Associated with More Bleeding After COVID Shots
The study’s main findings were that a heavier menstrual flow was more likely for those respondents who were of non-white race; were Hispanic/Latinx; were older; had a diagnosed reproductive condition; used hormonal contraception; had been pregnant in the past (whether or not they had given birth); were parous (had given birth), or had experienced fever or fatigue after vaccination. A higher proportion of respondents with reproductive conditions often associated with altered bleeding (endometriosis, painful periods, menorrhagia, fibroids, polycystic ovarian syndrome and adenomyosis) experienced a heavier menstrual flow than those without diagnosed reproductive conditions.
The combination of a reproductive history that includes having been pregnant but not giving birth in the past was associated with the highest risk of heavier flow after a COVID shot, although having given birth (parity) was also associated with heavier flow. The type of COVID, race, and use of hormonal contraception were not significant factors.2
In the regularly menstruating group, 43.6 percent reported no change in menstrual flow. However, while 14.3 percent reported a mixture of light or no change, 42.1 percent reported heavier menstrual flow following vaccination.3
Non-Menstruating Respondents Report Breakthrough Bleeding
The non-menstruating group consisted of those over age 55, who were postmenopausal and had not bled for at least twelve months before vaccination, as well as those who were premenopausal and using long acting (LARC) or continuous hormonal contraceptives and/or gender-affirming hormones that eliminates menstruation. Among those who were postmenopausal and not on any hormonal treatments, breakthrough bleeding was reported by 66 percent of respondents.
Age was a factor in postmenopausal breakthrough bleeding, with those who were younger (mean = 59.8 years) experiencing it more often than those who did not (mean = 61.4 years). Non-Hispanic individuals were also less likely to experience breakthrough bleeding.
Just over 70 percent of non-menstruating respondents using LARC reported breakthrough bleeding, while 38.5 percent of those using gender-affirming hormones reported changes.
Unexplained Menstrual Bleeding May Lead to Diagnostic Tests
Increased bleeding associated with menstruation is often distressing and may require providers to perform diagnostic procedures to investigate its origins. This is especially true after menstruation has stopped, such as during post menopause, and when breakthrough bleeding occurs, which could be an early sign of cancer.
Responsiveness to external stressors can affect menstruation and is one reason menstrual cycles are seen as reflecting overall health status, or a so-called “vital sign” in clinical practice.4 If potential side effects of a medical treatment like a vaccine are not widely shared with doctors and patients, it can lead to unnecessary, painful, and expensive diagnostic procedures such as uterine biopsy.
Study authors pointed out that the uterine reproductive system is flexible and adaptable in the face of stressors, which allows it to weather short term challenges in a way that leaves long-term fertility intact. Running a marathon, short-term calorie restriction and psychosocial stressors, for instance, can correspond to menstrual cycle irregularity. An immunological challenge like a vaccination or an illness can cause systemic inflammation in the body that provokes immune mediated responses in organs, including the uterus, and inflammation influences ovarian hormones.5
Vaccine Effects on Changes in Menstruation Not Well Studied
Menstruation is seldom considered a variable during vaccine trials, aside from researchers determining the clinical trial participant’s last menstrual period as part of established protections against volunteers being or getting pregnant. The vast majority of research that has been conducted regarding reproductive and menstrual function centers around whether live attenuated vaccines are safe to give to someone who is pregnant or whether vaccination affects fertility. The research that has been conducted on menstrual cycles specifically is often unable to establish a causal link, as the data are obtained through retrospective surveys, data mining and randomized controlled trials, which often do not allow a mechanism for reporting these menstrual changes.6
New vaccine trial protocols do not typically monitor for acute adverse events for more than seven days, and additional follow-up communications by researchers rarely—if ever—inquire about menstrual cycles or bleeding. Therefore, manufacturers have not systematically investigated the extent to which changes in menstruation is a coincidence or a potential side effect of the vaccine being investigated.
In media coverage, medical doctors and public health experts often hasten to say that there was “no biological mechanism” or “no data” to support a relationship between vaccine administration and menstrual changes. In other cases, experts declared that these changes were more likely a result of “stress.”7 Dismissal by medical experts of menstrual irregularities after vaccination can fuel even greater public concern.
Changes in Fertility After COVID Shots Difficult to Assess
Concerns about changes in fertility following COVID shots have been difficult to assess because many women, who were enrolled in the pre-Emergency Use Authorization (EUA) clinical trials, were using contraception and, therefore, pregnancy rates were extremely low.8 Pfizer and Moderna advise those who are pregnant when they receive mRNA COVID products, to enroll in a pregnancy registry to help monitor outcomes in women exposed to their products during pregnancy.9 10 Both manufacturer product inserts state that available data on their products “administered to pregnant women are insufficient to inform vaccine-associated risks in pregnancy.”11 12
Pfizer and Moderna report that safety of the mRNA biologics when given to pregnant women was evaluated using toxicity studies in rats injected with the mRNA product twice before mating and twice during gestation. The companies said there were no vaccine-related fetal malformations or variations and no adverse effects on postnatal development observed in the rats.13 14
Boston University’s Pregnancy Study Online (PRESTO) may help to evaluate changes in menstrual patterns and fertility after women receive mRNA shots. PRESTO is a prospective cohort study initiated in 2013 that recruits women and their male partners, who are actively trying to conceive but are not yet pregnant. The study evaluates risk factors for delayed conception, miscarriage and adverse birth outcomes, and is the first internet-based fertility study in the United States.15
PRESTO study director and professor of epidemiology Lauren Wise, ScD said, “The prospective design is really helpful, because what we do is we only compare women before and after they received the vaccination. So, it’s not like we’re enrolling people who have already had a vaccination and the menstrual changes into our study. We would only be looking at their data comparing before the vaccination versus after. And so that’s a good way to guard against any bias in the reporting of the menstrual factors or the selection of the participants into the study.”16
Gender Bias Still Prevalent in Clinical Trials
The frightening legacy of Thalidomide, an anti-nausea drug given to pregnant women in the 1950’s caused fetal malformations, as well as fear of harming women’s fertility, made drug makers leery of testing drugs in women and, particularly, in pregnant women. In 1993, the NIH passed the Revitalization Act, requiring researchers they funded to include both women and men in clinical studies, and to analyze their results by sex or gender. While experts say the gaps in science have narrowed, the system is still failing women. (For instance, research on hormonal contraception methods’ mental health effects is scarce, even though some studies suggest a link with depression.)17
The missing research makes it difficult for women to make confident health decisions, especially around their sexual and reproductive health. As a result, women may risk adverse effects from an approved drug that was untested in women or suffer unnecessarily with health problems that just don’t occur in men. “We’re constantly having to play experiments in our own bodies because nobody tested the drugs on us, so we have to play musical pills with anything we take,” says Sarah Hill, PhD, a professor at Texas Christian University.18
Global Studies of Menstruation Changes After COVID Shots Show Similar Results
Several other studies on COVID vaccination and menstrual changes have reported results similar to the most recent University of Illinois study. A study of 2269 menstruating women in the North Africa and Middle East region published in March 2022 reported that 66.3 percent reported post-vaccination abnormalities.19
An Italian study also published in March 2022 included 169 women of childbearing age, who had not experienced cycle irregularities for the past year, did not have gynecological or non-gynecological disease, and who were not receiving hormonal or non-hormonal treatments before they were vaccinated. They reported that 50–60 percent of reproductive age women who received the first dose of COVID-19 vaccine had menstrual cycle irregularities, regardless of the type of vaccine administered, with the occurrence of menstrual irregularities slightly higher (60 to 70 percent) after the second dose, which suggested a potential challenge/re-challenge effect. After both the first and second doses of vaccine, the most common changes were longer and heavier menstrual cycles than usual or expected.20
Another study using data from the fertility tracking app Natural Cycles compared three menstrual cycles pre- and post-vaccine for the vaccinated group and six cycles total for the unvaccinated group. Researchers found a .71-day cycle increase in cycle length following a first dose of mRNA product and a .91-day cycle increase following a second dose. There were no changes in cycle length in the unvaccinated group.21
NIH Awards Grants to Study Effects of COVID Shots on Menstruation
In August 2021, the NIH awarded one-year supplemental grants totaling $1.67 million to five academic institutions to explore potential links between COVID shots and menstrual changes. Researchers at Boston University, Harvard Medical School, Johns Hopkins University, Michigan State University and Oregon Health and Science University will conduct the research.22
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Click here to view References:
1 Lee KMN, Junkins EJ et. al. Investigating trends in those who experience menstrual bleeding changes after SARS-CoV-2 vaccination Science Advances July 15, 2022.
8 Male V. Menstruation and covid-19 vaccination The British Medical Journal Jan. 26, 2022.
9 U.S. Food and Drug Administration. Pfizer/COMIRNATY package insert.
10 FDA. Moderna/SPIKEVAX package insert.
11 FDA. Pfizer/COMIRNATY package insert.
12 FDA. Moderna/SPIKEVAX package insert.
13 FDA. Moderna/SPIKEVAX package insert.
14 FDA. Pfizer/COMIRNATY package insert.
15 Boston University Slone Epidemiology Center. Pregnancy Study Online (PRESTO).
16 McAlpine K. Question of the Week Podcast: Do covid vaccines affect the menstrual cycle? Boston University.
17 Pattillo A. Sexist Science Makes Women’s Reproductive Health a Guessing Game Inverse Nov. 16, 2019.
19 Muhaidat N, Alshrouf MA et al. Menstrual Symptoms After COVID-19 Vaccine: A Cross-Sectional Investigation in the MENA Region International Journal of Women’s Health Mar. 28, 2022.
20 Lagana SA, Veronesi G et al. Evaluation of menstrual irregularities after COVID-19 vaccination: Results of the MECOVAC survey Open Access Mar. 9, 2022.
21 National Institutes of Health. COVID-19 vaccination associated with a small, temporary increase in menstrual cycle length, suggests NIH-funded study Jan. 6, 2022.
22 National Institute of Child Health and Human Development. Item of Interest: NIH funds studies to assess potential effects of COVID-19 vaccination on menstruation Aug. 30, 2021.