Flu Vaccines May Slightly Disrupt Your Menstrual Cycle
Based on facts, either observed and verified firsthand by the reporter, or reported and verified from knowledgeable sources.
Newsweek AI is in beta. Translations may contain inaccuracies—please refer to the original content.
Getting a flu shot may affect your menstrual cycle, a study has revealed—although the change is temporary and typically very minor.
Researchers at Oregon Health & Science University found that women who received the influenza vaccine experienced a small increase in the length of their menstrual cycle—on average, less than a single day.
The full findings of the study were published in the journal JAMA Network Open.
What Is the Flu Shot?
The influenza (flu) vaccine is a seasonal injection designed to protect against the most common strains of the flu virus each year.
According to the U.S. Centers for Disease Control and Prevention (CDC), the vaccine helps your body build immunity, reducing the severity of illness and lowering the risk of flu-related complications.
A stock image showing a woman tracking her menstrual cycle.
A stock image showing a woman tracking her menstrual cycle.
Antonio_Diaz/iStock / Getty Images Plus
What Did the Study Find?
The researchers analyzed data on 1,501 individuals who reported regular menstrual cycles. Of those, 791 received the flu shot alone, while 710 received both the flu and COVID-19 vaccines at the same time. Most participants were under 35, lived in the U.S. or Canada and held at least a college degree.
Both groups experienced a small, temporary increase in cycle length:
Flu shot only : An average increase of 0.4 days
: An average increase of 0.4 days Flu + COVID-19 shot: An average increase of 0.49 days
These changes were most noticeable in people who were vaccinated during the first half of their menstrual cycle—the follicular phase. Those vaccinated during the second half—the luteal phase—did not experience any change.
Notably, a small subset of participants (around 5 percent) experienced a more noticeable shift of eight days or more in their cycle length. However, menstrual cycles returned to normal by the next cycle in all cases.
Why This Matters
Though the changes were minor and temporary, they highlight an important aspect of public health: transparency and trust.
Menstrual health is a routine and significant part of life for many people. A sudden or unexplained change—especially following vaccination—can understandably spark anxiety, particularly around fertility.
While the slight shift found in this study isn't cause for medical concern, it can still have a meaningful emotional and practical impact on those who experience it.
This research helps address a critical gap in vaccine science. For decades, there has been little to no data on how vaccines, like the flu shot, affect menstrual cycles. The COVID-19 pandemic shed light on this issue, as many people reported changes in their cycles post-vaccination. This study builds on that work by showing that even long-established vaccines like the flu shot can have a temporary effect.
Concerns about how vaccines might affect the menstrual cycle aren't new—and Japan offers a striking example of how these fears can have lasting consequences.
In 2013, the Japanese government suspended its HPV (human papillomavirus) vaccination program after reports emerged that some young women were experiencing menstrual changes and other side effects. The concern was that the vaccine might harm fertility, even though there was no solid scientific evidence to support this.
Before the scare, about 70 percent of eligible girls were getting the HPV vaccine. After the reports spread, that number plummeted to less than 1 percent. Even now, over a decade later, vaccination rates remain low in Japan, and trust in the program hasn't fully recovered.
The real-world impact has been devastating: Experts estimate that around 10,000 preventable deaths from cervical cancer could occur in Japan over the next 50 years as a result of this drop in vaccine uptake. Other countries' HPV programs were also negatively affected due to the international spread of vaccine fears.
Do you have a tip on a science story that Newsweek should be covering? Do you have a question about asteroids? Let us know via science@newsweek.com.
Reference
Boniface, E. R., Darney, B. G., van Lamsweerde, A., Benhar, E., Alvergne, A., & Edelman, A. (2025). Menstrual cycle length changes following vaccination against influenza alone or with COVID-19. JAMA Network Open, 8(4), e257871. https://doi.org/10.1001/jamanetworkopen.2025.7871
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