In the vast landscape of public health research, one domain has long lingered in the shadows: menstrual health outcomes in relation to vaccination. Historically excluded from vaccine trials and routinely overlooked in drug development, menstrual cycle changes have rarely been part of the scientific discourse. But this is changing. A groundbreaking international study led by Oregon Health & Science University has unearthed compelling evidence that receiving the influenza vaccine—either alone or in combination with a COVID-19 vaccine—can produce a small, temporary shift in menstrual cycle length. Though the change is less than a day on average, it offers critical insight into the intersection between immunization and reproductive health, bringing long-ignored questions to the forefront of scientific exploration.
Why Menstrual Health Was Left Out of Vaccine Research
For decades, menstrual health has been treated as peripheral to the study of medical interventions. Despite its fundamental importance to nearly half the global population, menstruation has been excluded from vaccine safety trials and dismissed as a “non-essential” outcome. But periods are more than monthly inconveniences—they reflect the body’s delicate hormonal balance, metabolic function, and immune response. The oversight of menstrual changes in clinical trials has meant that countless women around the world have encountered unexpected shifts in their cycles without a scientific explanation, often relying on anecdotes, forums, and social media rather than data and clinicians.
The consequences of this neglect are far-reaching. During the COVID-19 pandemic, millions of individuals noticed changes in their menstrual cycles after receiving vaccines. With no prior framework to understand these shifts, fear and misinformation quickly filled the void. Vaccine skepticism surged, fueled in part by speculation that immunization could affect fertility. Without data to counter the rumors, public trust in vaccines suffered a serious blow.
The Pandemic as a Natural Experiment in Public Health
Unintentionally, the global COVID-19 vaccination campaign created one of the largest uncontrolled public health experiments in history. With billions of people receiving vaccines within a relatively short time frame, researchers saw a rare opportunity to investigate questions previously ignored—chief among them, how vaccines might influence the menstrual cycle.
Although COVID-19 vaccines were not designed with menstrual outcomes in mind, the shared experience of immunization across such a massive population sparked a wave of anecdotal reports: delayed periods, heavier bleeding, skipped cycles. Some individuals worried these changes might signal long-term reproductive harm. These fears, though scientifically unfounded, had real consequences. Vaccine hesitancy ballooned. Misinformation flourished. And once again, the scientific community found itself playing catch-up.
Turning Anecdote Into Evidence: The Study Design
To separate fact from fiction, researchers conducted a retrospective global cohort study titled “Menstrual Cycle Length Changes Following Vaccination Against Influenza Alone or With COVID-19”, published in JAMA Network Open. This wasn’t a traditional clinical trial—it was an observational study using data from a digital birth control app used by individuals across the globe.
The researchers focused on 1,501 menstruating individuals between the ages of 18 and 45 who had consistent cycle lengths (between 24 and 38 days) across three cycles before vaccination and who were not using hormonal contraception, which could confound results. Among them, 791 had received the influenza vaccine alone, while 710 had received both the flu and COVID-19 vaccines on the same day.
Crucially, each person served as their own control. By comparing cycle length during the vaccination month to each individual’s average across the three prior months, researchers could isolate the impact of the vaccine more precisely.
The Numbers Tell a Subtle But Significant Story
The results offered a mixture of reassurance and intrigue. On average, individuals who received the influenza vaccine alone experienced an adjusted increase in cycle length of just 0.40 days. For those who received both the flu and COVID-19 vaccines simultaneously, the increase was 0.49 days. These changes, while statistically significant, were well within the range of natural variability.
Furthermore, in the cycle following vaccination, menstrual lengths returned to baseline levels. This reversal was crucial—it suggested that any vaccine-related changes were temporary and self-correcting. Even among the small percentage (under 6%) who experienced an increase in cycle length of eight days or more—a threshold deemed clinically meaningful—most returned to normal by the next cycle.
Importantly, no significant difference was observed between those who received only the flu shot and those who got both flu and COVID-19 shots. P-values above 0.05 confirmed that the observed differences could likely be due to chance rather than a true distinction between the two groups.
Why Timing Within the Cycle Matters
One of the study’s most striking revelations was the role of vaccination timing within the menstrual cycle. Researchers divided participants into two groups based on whether they received their shots during the follicular phase (the first half of the cycle, leading up to ovulation) or the luteal phase (the second half, post-ovulation).
Here, the data painted a clearer picture: individuals vaccinated during the follicular phase experienced more noticeable changes. Specifically, those who received only the flu vaccine during this phase saw a cycle length increase of 0.82 days. Those who received both vaccines experienced an increase of 0.99 days. In contrast, those vaccinated during the luteal phase showed no statistically significant change at all.
This pattern aligns with previous findings suggesting the follicular phase is more sensitive to immune activation. The immune system and reproductive system are deeply intertwined—cytokines and inflammatory signals play key roles in both pathogen defense and the cyclical breakdown and rebuilding of uterine tissue. A vaccine-triggered immune response may therefore temporarily alter hormonal cues or uterine processes, especially during the follicular phase.
Small Changes, Big Implications
So what does a half-day shift in cycle length actually mean? On an individual level, it might seem trivial. But on a population level, the implications are profound. These findings confirm that vaccines can have a subtle, time-limited effect on reproductive physiology—an effect that is measurable, predictable, and most importantly, non-harmful.
By identifying the menstrual phase as a moderating factor, the study offers new tools for clinicians and patients alike. Individuals concerned about vaccine-related menstrual changes can choose to schedule shots during the luteal phase, potentially avoiding any noticeable cycle effects. Meanwhile, scientists can begin integrating menstrual data into broader vaccine research, bridging the gender gap in clinical evidence.
The Path Forward: Trust Through Transparency
Perhaps the most important legacy of this study is not the data itself, but what it represents: a shift toward scientific transparency and inclusivity. By listening to people’s experiences, validating their observations, and rigorously testing the underlying biology, researchers have taken an important step in rebuilding public trust.
This work is especially vital in the current climate. Vaccine hesitancy remains a formidable obstacle to global health, driven in part by fears that have gone unaddressed. The World Health Organization has named it one of the top threats to public health. Misinformation, especially regarding reproductive health, spreads easily when data is scarce. By producing clear, peer-reviewed evidence on vaccine effects on menstruation, studies like this one help to restore credibility.
Menstruation Deserves a Seat at the Scientific Table
The menstrual cycle is a dynamic biological process influenced by hormones, the immune system, metabolism, and stress. It reflects the overall health of the body in ways scientists are only beginning to understand. Ignoring it in vaccine trials and clinical research is not just a scientific oversight—it’s a social and ethical one.
With digital health tools and wearable trackers becoming more common, the barriers to collecting menstrual data are rapidly disappearing. There is no longer a logistical excuse to exclude menstruation from research. Instead, menstrual cycle data should be embraced as a powerful tool for understanding broader physiological effects.
Final Thoughts: A New Era of Reproductive-Inclusive Research
The Oregon-led study may be small in scope, but it signals a paradigm shift. For too long, women’s health outcomes have been relegated to the margins of medical research. By bringing menstrual cycle changes into the spotlight, this work opens the door to a more inclusive, comprehensive approach to studying the body.
Vaccines remain one of the greatest achievements in modern medicine. They have saved millions of lives and continue to be our best defense against infectious disease. But understanding their effects on every part of the human system—including the reproductive system—is vital to ensuring that all individuals can trust and benefit from them fully.
As we move forward, menstrual health must no longer be an afterthought. It is not a side effect—it is a signal. And listening to it may unlock a new era of precision public health, one that truly serves everyone.
Reference: Emily R. Boniface et al, Menstrual Cycle Length Changes Following Vaccination Against Influenza Alone or With COVID-19, JAMA Network Open (2025). DOI: 10.1001/jamanetworkopen.2025.7871
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