Flu Shot Tied to Brief Menstrual Cycle Disruptions

 

• Influenza vaccination with or without concurrent COVID-19 vaccination was linked with longer menstrual cycles.
• Most menstrual cycles returned to pre-vaccination durations in the cycle after vaccination.
• Longer menstrual cycles were more likely when vaccination happened during the follicular phase.

Women's menstrual cycles temporarily became a little longer after receiving a flu shot, regardless of whether they also received a COVID vaccine at the same time, a retrospective study found.

Among 1,501 women with regular menstrual cycles, those who received a flu shot alone saw their average menstrual cycle significantly increase by 0.40 days (95% CI 0.08-0.72) compared with pre-vaccination durations, while women who received a flu and COVID shot together had their cycle lengthened by an additional 0.49 days (95% CI 0.16-0.83). The difference between groups was not significant.

In the subsequent cycle, neither group of women had significantly longer cycle lengths on average versus their pre-vaccination durations, reported Alison Edelman, MD, MPH, of Oregon Health & Science University in Portland, and colleagues in JAMA Network Openopens in a new tab or window.

A clinically meaningful change of at least 8 additional days of menstruation was observed among a small number of women during the initial cycle in which they were vaccinated -- both with the flu shot alone (4.7%) or with the two vaccines together (5.9%). Among these individuals, 27.8% and 20.5%, respectively, continued to have their cycles extended by at least 8 days in their subsequent menstrual cycle.

"Our findings about flu vaccination are very similar to what we have observed following vaccination for COVID-19 aloneopens in a new tab or window," co-author Emily Boniface, MPH, also of Oregon Health & Science University, told MedPage Today. "We saw a small increase in menstrual cycle length -- less than 1 day -- which returns to pre-vaccination cycle patterns in the subsequent cycle."

Like in the prior COVID study, Edelman and colleagues found that only those who were vaccinated in the follicular phase before ovulation saw their cycle length significantly increase. Follicular-phase vaccination was associated with 0.82 extra cycle days (95% CI 0.40-1.24) with a flu shot only and 0.99 more days (95% CI 0.55-1.43) with concurrent flu and COVID-19 vaccinations. Those vaccinated during the luteal, post-ovulation phase saw no significant change in cycle length.

"These findings can help clinicians to reassure their patients that any menstrual length changes that they might experience following vaccination are temporary and unlikely to be clinically concerning," Boniface said. "Clinicians can also counsel patients who wish to avoid these changes to time the receipt of their vaccine with the later part of their cycle, i.e., in the luteal phase following ovulation."

The study results are entirely in line with research on COVID-19 vaccination and menstrual cycles, explained Laura Payne, PhD, of Harvard Medical School and McLean Hospital in Boston. The evidence points to longer menstrual cycles not being a long-term problem with flu vaccination, nor are there issues with fertility.

"What I think is so interesting is that the flu vaccine by itself also produced those changes," Payne told MedPage Today. "This shows there is some inflammation, immune system, and sex hormone relationship that exists probably across many vaccines."

For their study, the researchers examined prospectively collected menstrual cycle data from the Natural Cycles digital birth control application, which helps users plan or prevent pregnancy without hormonal contraceptives. The study included app users ages 18 to 45 years who contributed data on flu and COVID-19 vaccinations.

Participants had to have reported menstrual cycle information from at least four consecutive cycles from April 2023 to February 2024: three pre-vaccination cycles and the cycle when they were vaccinated. When available, the researchers included data from the cycle immediately after vaccination.

The study's primary outcome was the adjusted change in an individual user's menstrual cycle length between the average of the three pre-vaccination cycles and the vaccination cycle.

The global study included 1,501 individuals, 791 who only received the influenza vaccine and 710 who were vaccinated for both influenza and COVID-19 on the same day. Among the participants, 82% were younger than 35 years, and 62.5% were located in the U.S. or Canada. Those who received both vaccines were more likely to be older than 30 years (55.1%) and more likely to be located in the U.S. or Canada (72%).

Study limitations included a large amount of missing data for several sociodemographic characteristics, the investigators noted. The study population was also primarily white, highly educated women with a low body mass index and no prior childbirths, which could limit the ability to generalize the study results.

Disclosures

The National Institute of Child Health and Human Development supported the study.

Boniface had no conflicts of interest. Edelman reported relationships with the NIH, Organon, HRA Pharma, FHI 360, the Gates Foundation, the World Health Organization, and CDC. Other co-authors reported relationships with the American College of Obstetricians and Gynecologists, the Society of Family Planning, Natural Cycles Nordic AB, and the journal Contraception.

Payne disclosed relationships with Bayer Healthcare, Mahana Therapeutics, Oregon Health and Science University, Brightside Health, and Pacific Rehabilitation Centers.

Primary Source

JAMA Network Open

Source Reference: opens in a new tab or windowBoniface ER, et al "Menstrual cycle length changes following vaccination against influenza alone or with COVID-19" JAMA Netw Open 2025; DOI: 10.1001/jamanetworkopen.2025.7871.

https://www.medpagetoday.com/infectiousdisease/vaccines/115337