Portland, OR—Research earlier this year raised the possibility that COVID-19 vaccines had an effect on menstrual cycles.

The report released in January and later published in Obstetrics & Gynecology noted that women receiving one dose of a COVID-19 vaccine during a single menstrual cycle had an increase in cycle length of nearly 1 day, compared with unvaccinated women, according to a study funded by the National Institutes of Health. Researchers from Oregon Health & Science University, Portland, and colleagues found that the increase in cycle length—a longer time between bleeding—was not associated with any change in the number of days of menses.

The authors provided reassurance that menstrual cycles vary somewhat from month to month and that the vaccine-induced increase fell within the range of normal variability. They called for additional research on how COVID-19 vaccination could potentially influence other menstrual characteristics, such as associated symptoms (pain, mood changes, etc.) and characteristics of bleeding (including heaviness of flow).

Now, a large international study has confirmed the findings of that study. The new study includes data from nearly 20,000 women from Canada, the United Kingdom, the United States, Europe, and other parts of the world who received any of nine different vaccines. Results were published in the British Medical Journal.

Aware of concerns about menstrual-cycle disruptions after COVID-19 vaccination, the researchers sought to find out more about that by conducting a global retrospective cohort study of prospectively collected data. Participants were international users of the menstrual-cycle tracking application Natural Cycles.

The focus was on 19,622 women aged 18 to 45 years with cycle lengths of 24 to 38 days and consecutive data for at least three cycles before and one cycle after. The participants were divided into two groups: 14,936 who were vaccinated and 4,686 unvaccinated who had at least four consecutive cycles over a similar time.

Researchers calculated the mean change by vaccination group for cycle and menses length, i.e., mean of three cycles before vaccination to the cycles after the first and second dose of vaccine and the subsequent cycle.

Most of the women (80.08%) were younger than age 35 years; 31.7% were from the UK, 28.59% from the U.S. and Canada, and 33.55% from Europe. Two-thirds of those vaccinated received the Pfizer-BioNTech (BNT162b2) COVID-19 vaccine, while 17.46% received Moderna (mRNA-1273), 9.06% received Oxford-AstraZeneca (ChAdOx1 nCoV-19), and 1.89% received Johnson & Johnson (Ad26.COV2.S).

Results indicate that women who were vaccinated had a less than 1day adjusted increase in the length of their first and second vaccine cycles, compared with those who were not vaccinated (0.71-day increase [99.3% CI 0.47-0.96] for first dose; 0.56-day increase [0.28-0.84] for second dose).

The authors added, however, that the adjusted difference was larger in women who received two doses in a cycle (3.70-day increase [2.98-4.42]).

"One cycle after vaccination, cycle length was similar to before the vaccine in individuals who received one dose per cycle (0.02-day change [99.3% confidence interval 0.10 to 0.14]), but not yet for individuals who received two doses per cycle (0.85-day change [99.3% confidence interval 0.24 to 1.46]) compared with unvaccinated individuals," they recounted. "Changes in cycle length did not differ by the vaccine's mechanism of action (mRNA, adenovirus vector, or inactivated virus). Menses length was unaffected by vaccination."

The researchers concluded that COVID-19 vaccination is associated with a small and likely temporary change in menstrual cycle length but no change in menses length.

The authors suggested that pharmacists and other vaccine providers can use their results to counsel menstruating women about what to expect with COVID-19 vaccinations. They also urged that menstrual-cycle data be collected during the development of future vaccines.

Background information in the article notes that a range of menstrual-cycle changes after COVID-19 vaccination have been reported, including longer and shorter cycles, missed cycles, heavier and lighter menstrual flow, and intermenstrual spotting. Clinical trials leading to the use of COVID-19 vaccines did not collect outcomes related to the menstrual cycle, however.

While the study findings suggest that the changes in the menstrual cycle were short-lived, the authors advised that "Even small changes, when unanticipated, can have a large adverse impact on the quality of life of people who menstruate and who experience episodes of social embarrassment, anxiety related to uncontained bleeding or fertility planning or prevention, and worry about what bleeding changes mean for their overall health. The absence of evidence about vaccines and menstrual health coupled with the long-standing sex-specific research inequities can also be interpreted by the public as a dismissal from the scientific and medical community."

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