Studies show COVID-19 vaccine affects menstrual cycles

Assessing the effects of COVID-19 vaccines on menstrual cycles is important, because the menstrual cycle is a significant indicator of women’s health, according to the authors of a review published in BMJ Sexual & Reproductive Health, who found most studies show the vaccine affects cycles.

Although women comprised about half of the participants in the original COVID-19 vaccine trials, no data were collected on how the shots affected their menstrual cycles.

Soon after the shots were rolled out, many women started reporting longer periods and heavier-than-normal bleeding, and many women who did not normally menstruate — including women on long-acting contraceptives and post-menopausal women — also reported unusual bleeding.

Tens of thousands of women reported symptoms to researchers and medical regulators in the U.S. and the United Kingdom respectively by mid-2021.

At the time, women’s concerns were often “blown off” and they felt “gaslighted,” Dr. Alison Edelman, one of the review article authors, told NBC.

Researchers called for studies into the issue, in part because they said disrupted menstrual cycles were driving “misinformation” that the vaccines were dangerous and fueling “vaccine hesitancy.”

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For the BMJ review, researchers from Harvard, Boston University, Michigan State University and Oregon Health & Science University surveyed and summarized the existing published literature in the PubMed database — which contains peer-reviewed research in the biomedical and life sciences literature — on the COVID-19 vaccines and menstruation.

“Overall, data from published studies indicate small transient changes in menstrual cycle length (ie, longer cycle length) following vaccination,” they concluded.

“Additionally, there is some evidence that other menstrual characteristics such as menstrual pain, menstrual flow and intermenstrual bleeding also occur following vaccination.”

Because there is more limited research, less is known for certain about the shot’s effects on specific age groups, such as adolescents or post-menopausal women. However, data suggest that breakthrough bleeding or menstrual cycle changes affect them, they wrote.

However, the lack of standardized measures for assessing menstrual-related issues makes it challenging to summarize the data, they wrote, so many of the studies did not necessarily measure the same outcomes.

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The researchers identified 53 studies on vaccination and menstruation published before Oct. 31, 2023.

These included 11 prospective cohort studies, which observed a group of women over time to see the effects of the shot, 11 retrospective cohort studies, which looked backward after the fact at the effects of the shot on a group of women and 31 cross-sectional or retrospective case-control studies, which looked backward at the effects of the shot by comparing women who experienced a symptom to those who didn’t.

They evaluated the papers for risk of bias and summarized the findings according to the primary ways the COVID-19 vaccine disrupted women’s menstrual cycles.

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First, they found with certainty that “the COVID-19 vaccine is associated with changes in cycle length, at least in adult populations,” noting that adolescent girls were more difficult to study.

Most of the studies, which had focused on cycle length because it is typically a well-tracked and easily defined outcome, found the shot was associated with a longer cycle.

Studies investigating cycle irregularities other than length were fewer and more mixed, the researchers reported. Some of these showed the shots had short-term effects and others revealed issues such as missed periods and intermenstrual bleeding, with symptoms increasing with the second and then subsequent shots.

There were also studies examining the vaccines’ effect on menstrual flow. These data, they said, were easier to access because there are many period-tracking apps that women use to track their flows. However, the data for this metric were also contradictory.

They reported several studies, like one that analyzed data in an app, that didn’t identify a difference in the lighter and heavier flow days during menstruation, although they did find a heavier flow overall. However, several studies also found a change in flow following vaccination.

Although the researchers emphasized the mixed nature of the results for this metric, they conceded that the data showing the vaccines affect menstrual flow were compelling enough that the UK Medicines and Healthcare products Regulatory Agency and Pfizer and Moderna all list “heavy flow” as a vaccine side effect.

The researchers also found that existing studies show that between 20-40% of menstruating women experience menstrual pain following vaccination and that the pain is similar following both the first and second vaccination doses.

They also reported that women with endometriosis — a painful condition where the lining of the uterus grows outside of the uterus — had worse cycle abnormalities, including fatigue, pain and regularity disorders, than other women.

Several larger studies examining population data found a “slightly increased risk” of bleeding in postmenopausal women. One large study found the risk increased in the 16 weeks following vaccination. Another found an especially strong statistically significant risk following a third dose of the vaccine.

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The studies suggested that women using hormonal contraception were somewhat protected from the effects of vaccination on their menstrual cycle. For example, one study found that women taking hormonal contraception had a shorter delay in their cycle than those who weren’t taking it. The effects varied with different types of contraception.

Another study found women taking hormonal contraception had more breakthrough bleeding than women who weren’t taking it.

The researchers also noted that all of the studies were conducted with women who had regular menstrual cycles pre-vaccination because it is too difficult to discern whether irregularities among women who already have irregular periods are vaccine-related or not.