The effect of COVID-19 vaccination on menstrual cycles of adolescents and young adults: a systematic review and meta-analysis.

Background and study objective: The binding of the SARS-CoV-2 spike protein from vaccination can affect the menstrual cycle. Most studies have focused on women of reproductive age, with less attention given to adolescent and young women, despite their increased risk of heightened responses to vaccines. Thus, we consolidated evidence on menstrual changes after COVID-19 vaccination for adolescent and young women.

Methods: OVID MEDLINE, EMBASE and CINAHL databases were searched (January 2020-December 2024) for peer-reviewed studies on COVID-19 vaccination on menstruating people <25. Of 80 articles identified, 15 met the inclusion criteria after review by two independent reviewers. We estimated risk ratios (RR) and mean differences (MD) when data permitted. We assessed publication bias with funnel plots and evaluated heterogeneity using Cochran's Q, Galbraith plots, and I² statistic. Outcomes included any measured or perceived changes in menstrual cycles, in bleeding length, and in cycle length (i.e. length between the first day of bleeding of two periods).

Results: Among the 15 studies, 24,647 adolescents and young adults aged 12 to 25 were included. The summary effect measure showed no effect of vaccination on any menstrual change (RR:1.09; 95% CI: 0.84-1.42) and significant heterogeneity across studies (I² = 69%). There was, however, a greater risk of longer cycle length after vaccination (RR=1.17, 95% CI: 1.08, 1.27) and no heterogeneity between these two studies (I²=0%); however, when assessed continuously there was a non-significant increase in cycle length (MD=0.24 days, 95% CI: -0.34, 0.82). No studies assessed menses bleeding length in adolescent and young women. Funnel plots suggested no publication bias.

Conclusion: Although few studies were included, available data suggest that there may be increased risk of a longer menstrual cycle length in adolescents and young women, but no other changes were identified. Further data are needed.