Menstrual Cycle and Vaccine Timing Influence Minor Cycle Variations

Recent research conducted by an international team led by Oregon Health & Science University has revealed that receiving influenza vaccines, either alone or alongside COVID-19 vaccines, can lead to slight changes in menstrual cycle length, particularly when administered during the follicular phase of the cycle. Specifically, the study found a temporary increase of less than one day in menstrual cycle duration following vaccination.

Historically, menstrual health outcomes have been largely overlooked in vaccine trials, creating a knowledge gap about how vaccines might impact menstrual patterns. During the COVID-19 vaccination campaign, concerns regarding possible menstrual disturbances gained public attention, fueled by anecdotal reports and misinformation, which contributed to vaccine hesitancy—a major public health challenge acknowledged by the World Health Organization.

The study, published in JAMA Network Open, involved 1,501 women aged 18 to 45 who used digital birth control apps, had regular cycles between 24 and 38 days, and were not on hormonal contraception. Participants received either the influenza vaccine alone or both influenza and COVID-19 vaccines on the same day. Each participant's cycle length before vaccination was compared to the cycle during vaccination, allowing researchers to identify the effects of the vaccines.

Results indicated an average cycle length increase of 0.40 days with influenza vaccine alone and 0.49 days with both vaccines combined, but these changes were temporary with no persistent effects in subsequent cycles. Notably, the most significant changes occurred when vaccines were administered during the follicular phase, with increases of approximately 0.82 to 0.99 days, whereas vaccination during the luteal phase showed no notable effect.

Importantly, about 4.7% of women vaccinated with influenza alone and 5.9% with both vaccines experienced a clinically meaningful cycle change of eight days or more during the vaccination cycle. Despite these minor variations, the majority of women reverted to their usual cycle length shortly after vaccination.

These findings provide reassurance that COVID-19 and influenza vaccines are unlikely to cause sustained menstrual disturbances. The effects are minor, temporary, and influenced by the timing of vaccination within the menstrual cycle. This knowledge can help clinicians and women plan vaccination timing more effectively, potentially reducing perceived or actual menstrual disruptions.

In summary, vaccines may cause small, short-term changes in cycle length, especially if administered during the follicular phase. This research helps to address previous misinformation and supports the safety of influenza and COVID-19 vaccines concerning menstrual health.

Source: https://medicalxpress.com/news/2025-05-menstrual-phase-vaccine-linked-minor.html