Characteristics and health factors influencing menstrual changes after COVID-19 vaccination: A Spanish retrospective observational study in currently menstruating women.

BACKGROUND: There is increasing evidence that menstrual changes (MC) should be considered as an adverse event resulting from COVID-19 vaccination. However, the contributing health factors are still poorly understood. OBJECTIVES: The aim was to analyze the characteristics and factors influencing MC after the administration of the severe acute respiratory syndrome coronavirus 2 vaccine. DESIGN & METHODS: A retrospective observational study of currently menstruating women (N = 14,550) in Spain was conducted during the month of December 2021 using an online survey. Among others, general characteristics of the menstrual cycle, medical history, and adverse events following vaccination-including MC-were recorded. Bivariate logistic regression analysis was performed to determine the influencing factors. RESULTS: 45.0%-50.9% of respondents reported experiencing mostly transient MC after both doses of COVID-19 vaccine, including different spotting (41.6%-49.0%), longer periods (26.5%-29.5%) and/or heavier flow (28.7%-31.6%). Binary logistic regression analysis showed that some of the inter-individual factors that may be involved in this unexpected event are age (dose 2: adjusted odds ratio (aOR): 1.02; 95% confidence intervals (CI): 1.02-1.03), heavy menstrual flow (dose 1: aOR: 1.12; 95% CI: 1.00-1.25), use of short- (dose 1: aOR: 1.38; 95% CI: 1.24-1.54) to medium-term contraception (dose 1: aOR: 1.31; 95% CI: 1.09-1.57), number of previous pregnancies (dose 1: aOR: 1.11; 95% CI: 1.03-1.19), pre-existing diagnoses of certain clinical conditions-including endometriosis (dose 1: aOR: 1.33; 95% CI: 1.11-1.59)-and suffering from other vaccine adverse events. CONCLUSION: Currently menstruating women may experience MC after COVID-19 vaccination. Further research is warranted to address the influencing factors, considering their heterogeneity according to the geographical background of the target population. This kind of evidence could prove instrumental in the context of future viral outbreaks, helping healthcare professionals to provide scientifically up-to-date information to patients for making informed decisions regarding their well-being, particularly in societies where menstruation remains a taboo subject.