Systematic Review Links COVID-19 Vaccines to Short-Term Menstrual Changes, Calls for Deeper Research

 


A systematic review of 61 studies has found consistent reports of menstrual changes following COVID-19 vaccination, though the effects appear to be short-term and generally mild. The findings bring data-driven clarity to an issue that first drew widespread attention through anecdotal accounts on social media during the initial months of global vaccination campaigns.

Regulatory agencies have already acted on early safety signals. In October 2022, the European Medicines Agency required heavy menstrual bleeding to be listed as a possible side effect of Pfizer and Moderna vaccines. By June of that year, menstrual disorders made up nearly 30% of all vaccine-related reports from women in the European Union.

The review, conducted under PRISMA guidelines, analyzed studies across multiple vaccines, including Pfizer, Moderna, AstraZeneca, Johnson & Johnson, Sinopharm, Sputnik V, Sinovac, and others. Pfizer dominated the data, appearing in 79% of studies. The majority of research relied on self-reported questionnaires (85%), often distributed via social media, which allowed for large-scale participation but introduced possible bias.

Every study included reported a higher rate of at least one menstrual disturbance in the first cycle after vaccination. Cycle length was the most commonly tracked variable, examined in 54 of the 61 studies. Most reported small but measurable shifts, such as increases of less than a day per cycle on average. However, outliers existed, with some studies documenting delays of more than two days. Roughly 9% of participants reported longer cycles, while 7% reported shorter ones.

Changes in bleeding volume were also frequent. Forty-four studies examined this outcome, with many women reporting heavier periods after vaccination. One large cohort study calculated a relative risk of 1.61 for heavier bleeding in the cycle following vaccination compared to the cycle before. Importantly, these patterns generally normalized within months. Menstrual duration showed similar trends: in one dataset, average menses length rose from six to 6.5 days, a modest but statistically significant increase.

Other disturbances varied in prevalence. Increased pain or cramps were noted in about a third of studies, with relative risks as high as 1.62, though one study found 11% of participants actually reported less pain. Breakthrough bleeding, including intermenstrual and postmenopausal bleeding, appeared in 18 studies, with prevalence ranging from 1% to 19%. A Swedish study highlighted postmenopausal bleeding risk as highest after the third dose.

Risk was not evenly distributed across populations. Women with conditions such as endometriosis, PCOS, thyroid disorders, or a history of heavy periods were more likely to report changes. Stress, higher BMI, and smoking were additional risk factors. By contrast, hormonal contraceptive use was associated with a reduced likelihood of disturbances, cutting risk by roughly 27% to 50% depending on the study.

The timing of vaccination within the menstrual cycle may also influence outcomes. Vaccination during the follicular phase was linked to cycle lengthening, while one study suggested luteal phase vaccination with mRNA vaccines shortened cycles. Women who received two doses within a single cycle saw an average delay of 2.3 days in their next period.

Despite the breadth of data, researchers caution against drawing causal conclusions. Most studies were cross-sectional, with limited use of control groups, making it difficult to separate vaccine-related effects from the high background rate of menstrual variability. Reliance on self-reported data further complicates interpretation, as women experiencing disturbances may have been more likely to participate. Many studies also excluded those under 18, leaving gaps in understanding for younger populations.

Still, even temporary disruptions can have an outsized impact on quality of life. Researchers stress that these findings should not be dismissed, and they argue for better integration of menstrual health into vaccine safety monitoring. The biological mechanisms remain unclear, though one hypothesis is that the immune response to vaccination interferes with hormonal and inflammatory pathways that regulate menstruation.

The review concludes with calls for more rigorous longitudinal research, including studies that compare vaccine types, dosing intervals, and effects across vulnerable groups. Monitoring menstrual health before and after vaccination could help address ongoing questions, while recognizing menstrual changes as a meaningful safety endpoint may allow women to make better-informed health decisions.

Link to Study

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