A new cohort study revealed that both influenza vaccination alone and in combination with COVID-19 vaccination could be associated with small, temporary changes in menstrual cycle length, particularly when administered during the follicular phase.

In the global retrospective study, investigators at the Oregon Health & Science University and several international institutions analyzed data from 1,501 individuals with regular menstrual cycles who used a digital birth control application. They compared menstrual patterns before and following vaccination in two groups: 791 participants who received only the influenza vaccine and 710 who received both the influenza and COVID-19 vaccines concurrently.

"Individuals vaccinated for influenza alone experienced an adjusted mean increase of 0.40 (95% confidence interval, CI = 0.08–0.72) days, while those vaccinated concurrently for influenza and COVID-19 experienced a mean increase of 0.49 (95% CI = 0.16–0.83) days," the study authors reported. The difference between the two vaccine groups was not statistically significant (P = .69).

The investigators also found that a small percentage of the participants experienced more substantial menstrual cycle changes, with 4.7% of the influenza-only group and 5.9% of the combined vaccine group experiencing cycle length changes of at least 8 days. This difference was also not statistically significant (P = .28).

Notably, the menstrual cycle changes were only associated with vaccination during the follicular phase, not during the luteal phase. During the follicular phase, the influenza-only group experienced a mean increase of 0.82 days (95% CI = 0.40–1.24), whereas the concurrent vaccination group saw an increase of 0.99 days (95% CI = 0.55–1.43).

All of the participants returned to their typical menstrual cycle lengths in the cycle following vaccination, confirming the temporary nature of these changes.

"While small changes in menstrual health may not seem meaningful to many clinicians and scientists, any perceived impact in a routine bodily function linked to fertility can cause alarm and contribute to vaccine hesitancy," the study authors noted in their discussion.

The investigators suggested their findings could help clinicians counsel patients about what to expect following vaccination and potentially address vaccine hesitancy. They also noted that individuals concerned about potential menstrual cycle disturbances could consider timing their vaccination to coincide with their luteal phase.

This study provided data on how influenza vaccination might affect menstrual cyclicity, a topic that "has been largely ignored throughout the almost century-long history of influenza vaccines," according to the study authors.

Conflict of interest disclosures can be found in the study.

Source: JAMA Network Open