Do COVID-19 Vaccines Affect Menstruation?

Throughout the pandemic, regardless of the type of vaccine, people who menstruate took to social media to note irregularities in their menstrual cycles after being vaccinated against the COVID-19 virus. Adding to the growing body of research supporting this anecdotal evidence, data from a recent paper published in npj Digital Medicine showed that—albeit small—changes in menstruation were observed following COVID-19 vaccination.

Changes in the menstrual cycle after COVID-19 vaccination

In the npj Digital Medicine study, Gibson and colleagues analyzed menstrual cycle data manually logged into the Apple Health phone app by 9,652 menstruating participants. After investigating a total of 128,094 cycles from 8,486 vaccinated and 1,166 unvaccinated participants, the researchers concluded that menstrual cycles increased in length after vaccination (from 0.39 to 1.26 days over the norm). When vaccination occurred during the follicular phase (when an egg matures in the ovaries), participants experienced longer menstrual cycles than when they were vaccinated during the luteal phase (thickening of the uterine lining).

Participants vaccinated during the follicular phase cycled for 0.97 to 2.27 days longer, but when vaccinated during the luteal phase, their cycles stopped earlier or lasted 0.39 days longer than their regular cycles. Note, the authors estimated that the follicular phase occurred early to mid cycle—the cycle being 28 days. Similar findings were reported in another study published in Open Medicine earlier this year. In that study, Lagana et al. reported a longer period length when any COVID-19 vaccine was administered during the early luteal phase (which the researchers considered to be during days 14 to 28 in the cycle).

When the vaccine was administered within the first 14 days of the menstrual cycle (follicular phase), Lagana et al. mentioned that many people who menstruate reported cycle alterations (earlier period) or heavier bleeding, but these differences were not found to be statistically significant. Neither study addressed whether ethnicity or other factors contributed to COVID-19 vaccine-related menstrual irregularities.

"When vaccination occurred during the follicular phase, participants experienced longer menstrual cycles than when they were vaccinated during the luteal phase."

To address whether ethnicity or other factors were associated with COVID-19 vaccine-related menstrual changes, Lee and colleagues examined data from 39,129 surveyed menstruating people between the ages of 18 and 80 who had been fully vaccinated (at least two boosters) and had not previously contracted COVID-19. Published in Science in July, their study reported heavier menstrual flow in respondents who

• were of Hispanic or Latinx descent,
• experienced systemic vaccine reactions (e.g., fever, fatigue, or other),
• were diagnosed with a reproductive condition (e.g., uterine fibroids, polycystic ovary syndrome, or other),
• took hormonal contraception, or
• who had been previously pregnant regardless of having given birth.

Perhaps most interestingly, Lee et al. reported breakthrough bleeding in respondents that did not regularly menstruate—i.e., people who used long-acting reversible contraceptives, who received hormone replacement therapy, or who were postmenopausal.

COVID-19 vaccine safety and menstrual changes

Current vaccine trials do not collect menstrual cycle outcomes post-vaccination, and the Vaccine Adverse Event Reporting System generally does not gather information on menstrual cycle irregularities. Women are also less likely to report on menstrual cycles due to social stigma. In their Science paper, Lee et al. wrote that, “generally, changes to menstrual bleeding are not uncommon or dangerous, yet attention to these experiences is necessary to build trust in medicine.”

Overall, menstrual cycle irregularities resolved in less than a year after vaccination, suggesting that the COVID-19 vaccine caused temporary changes but did not lead to long-term impacts. Numerous studies have also reported on the safety of COVID-19 vaccines. 

"Other vaccines, such as immunizations against hepatitis B and HPV have also been noted to affect menstrual cycles."

Other vaccines, such as immunizations against hepatitis B and human papillomavirus, or HPV, have also been noted to affect menstrual cycles. Similar effects have been noted in women who have contracted COVID-19, but there are conflicting data.

Given that contracting COVID-19 is potentially life-threatening, vaccines remain the best defense to help protect against SARS-CoV-2 infection. But these studies do highlight an insidious issue within the medical and scientific community: the lack of gender-based studies. To date, regulatory bodies have not announced whether or not they will track menstrual irregularities in future clinical studies.