Flu and COVID-19 Vaccines Linked to Minor, Temporary Menstrual Cycle Changes: Study Offers Reassurance

"The study found that people with regular menstrual cycles experienced a slight, temporary change in cycle length after getting the flu vaccine, either alone or with a COVID-19 shot. The findings can help doctors reassure patients worried about menstrual side effects from vaccines," the researchers reported.

Several studies have reported a link between COVID-19 vaccination and menstrual disturbances, raising concerns among individuals about potential effects on menstrual health. However, limited data exist on how other vaccines, such as the influenza vaccine, might influence the menstrual cycle. Understanding these effects is important for guiding individuals on what to expect and addressing vaccine-related hesitancy. To explore this, Emily R. Boniface, Department of Obstetrics and Gynecology, Oregon Health & Science University, Portland, and colleagues conducted a study to assess whether receiving the influenza vaccine, either alone or along with a COVID-19 vaccine, is associated with changes in the length of the menstrual cycle.

For this purpose, the researchers conducted a global retrospective cohort study using data from English-speaking users of a digital birth control app, collected between April 25, 2023, and February 27, 2024. The study included individuals aged 18 to 45 years who were not on hormonal contraception and had consistent cycle lengths of 24 to 38 days across three consecutive cycles before vaccination. They assessed changes in menstrual cycle length following seasonal influenza vaccination, with or without a COVID-19 vaccine.

The primary outcome measured within-individual changes in cycle length, while the secondary analysis examined the menstrual phase at vaccination.

The study led to the following findings:

• A total of 1501 individuals were included in the study: 791 received only the influenza vaccine, while 710 received both the influenza and COVID-19 vaccines.
• Most participants were under 35 years old (82%), had at least a college degree (74.8%), and were from the US or Canada (62.5%).
• Racial and ethnic data were missing for 72.2% of participants. Among the reported, 24.5% were White, 1.3% identified as other, and small percentages identified as Asian, Hispanic or Latina, Black, American Indian/Alaska Native, or Middle Eastern/North African.
• Those who received only the flu vaccine had a small increase in menstrual cycle length by 0.40 days, while those who got both vaccines had an increase of 0.49 days. The difference between the two groups was not statistically significant.
• A change in cycle length of 8 days or more was observed in 4.7% of flu-only recipients and 5.9% of those who received both vaccines.
• In both groups, cycle lengths returned to normal in the cycle following vaccination.
• Changes in cycle length occurred only when vaccination took place during the follicular phase, not the luteal phase.

The study found that receiving the influenza vaccine alone or along with a COVID-19 vaccine was linked to a small, temporary change in menstrual cycle length, typically less than one day, especially when vaccination occurred during the follicular phase. There was no significant difference between vaccine groups in the number of individuals experiencing a meaningful cycle change of eight days or more.

"These findings offer reassurance that vaccination is not associated with major menstrual disruptions and may help clinicians address concerns about menstrual side effects while supporting vaccine confidence," the authors concluded.

Reference:

Boniface ER, Darney BG, van Lamsweerde A, Benhar E, Alvergne A, Edelman A. Menstrual Cycle Length Changes Following Vaccination Against Influenza Alone or With COVID-19. JAMA Netw Open. 2025;8(4):e257871. doi:10.1001/jamanetworkopen.2025.7871