Temporary Menstrual Cycle Shifts After Flu and COVID-19 Vaccines

Vaccine research covers many outcomes, but menstrual health isn’t usually one of them.

A new study conducted by researchers from Oregon Health & Science University investigates whether seasonal influenza vaccines, alone or given alongside a COVID-19 vaccine, cause changes in menstrual cycle length. The findings point to small, temporary shifts, but no long-term effects were observed.

The study was published in JAMA Network Open.

Menstrual health matters in vaccine research

Despite menstruation being a regular part of life for many, clinical trials have rarely considered how vaccines might affect menstrual cycles. With the rollout of COVID-19 vaccines, this gap gained renewed urgency. As people began sharing personal experiences of cycle changes, such as delays or heavier bleeding, questions about possible links between vaccination and menstruation became more widespread. These concerns, whether grounded in data or not, contributed to vaccine hesitancy.

This isn’t an isolated issue. A similar situation unfolded in Japan following reports of menstrual disturbances after the human papillomavirus vaccine. Public concern led to a dramatic fall in vaccine uptake, with lasting effects on health outcomes.

“Misinformation and the dearth of data to confirm or refute the vaccine experience can decrease acceptability and uptake of a vaccine,” said the authors.

Previous studies have found a small, short-lived increase in menstrual cycle length after COVID-19 vaccination. But whether other vaccines, like the widely recommended seasonal influenza vaccine, might have similar effects, hasn’t been well studied.

“While the COVID-19 pandemic brought many challenges, it did highlight the lack of evidence on this important patient-oriented outcome,” the authors commented.

Since the COVID-19 and flu vaccines are now commonly given together, understanding their combined impact matters. This new study aims to help fill that gap by examining whether flu vaccines – alone or with a COVID-19 shot – are linked to changes in cycle length.

Tracking cycle changes after vaccination

The researchers carried out a global retrospective cohort study, using prospectively collected data from a digital birth control app, Natural Cycles, which tracks menstrual cycles in real-time. This allowed the team to examine menstrual patterns before and after vaccination. The study included 1,501 individuals aged 18–45 years who were not using hormonal contraception and had consistent cycle lengths ranging from 24–38 days. Each participant contributed data for at least three cycles before vaccination and up to two after. Of these, 791 participants received the influenza vaccine alone, while 710 received both influenza and COVID-19 vaccines on the same day.

On average, the influenza-only group experienced an increase in cycle length of 0.40 days, while the group receiving both vaccines showed an increase of 0.49 days. In both groups, menstrual cycle lengths returned to baseline in the following cycle.

There was no statistically significant difference between the two groups, suggesting that receiving both vaccines on the same day does not lead to greater menstrual disruption than the flu vaccine alone.

“When we examined the adjusted change in cycle length in both vaccination groups by the menstrual phase of vaccination, only individuals who were vaccinated in the follicular phase experienced a statistically significant increase in cycle length compared with their prevaccination mean length,” said the authors.

No significant change was observed when vaccines were given in the luteal phase.

A small proportion of participants – under 6% in both groups – experienced a change of 8 days or more.

The results held after accounting for underlying conditions such as polycystic ovary syndrome or thyroid disorders.

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What this means for patients and public health

The small, temporary changes in cycle length – less than one day on average – suggest that influenza vaccines, whether given alone or alongside a COVID-19 shot, are unlikely to cause significant menstrual disturbances for most people. Importantly, the study found no evidence of lasting effects or changes linked to fertility.

From a clinical perspective, the results provide helpful context for patient conversation. With vaccination now a routine part of public health strategy, having data to address menstrual concerns can help build trust.

“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 authors noted.

Addressing these concerns directly may also help reduce vaccine hesitancy, particularly among those who are already uncertain.

The study also suggests a possible way to mitigate menstrual changes: vaccines given during the luteal phase (after ovulation) were not associated with significant cycle length changes, in contrast to those administered in the follicular phase. While further evidence is needed, this may offer a practical strategy for those looking to reduce potential disruptions.

Broader studies across more diverse populations are still needed.

“Any change, even if small and not clinically relevant, is important to the public,” the team added.

Reference: Boniface ER, Darney BG, Lamsweerde A van, Benhar E, Alvergne A, Edelman A. Menstrual cycle length changes following vaccination against influenza alone or with COVID-19. JAMA Network Open. 2025. doi: 10.1001/jamanetworkopen.2025.7871