Clear Majority of Female COVID Vax Recipients Report Changes in Menstrual Cycle Length or Regularity: British Medical Journal (BMJ)

A pre-print commentary (here) in the British Medical Journal (BMJ) found that COVID-19 vaccination is associated with an increased risk of longer menstrual periods during the first 6 months post-vaccination, especially for women who had irregular, short, or long pre-vaccination menstruation.

In the commentary, Italian researchers analyzed a study published in the American Journal of Obstetrics and Gynecology (AJOG) conducted by doctors from Harvard, Brigham and Women’s Hospital, and Michigan State University that found COVID vaccination was associated with a “change to longer cycles in the first 6 months after vaccination.”

The menstrual cycle changes occurred “particularly among women whose cycles were short, long, or irregular before vaccination,” the authors explained.

Read the full study:


The AJOG study observed 3,858 premenopausal women who completed “monthly and quarterly surveys related to the COVID-19 pandemic between April 2020 and November 2021,” including “self-reported data about positive SARS-CoV-2 test, COVID-19 vaccination status and vaccine type.”

In those surveys, the respondents were “asked about menstrual cycle length and regularity before ‘before COVID-19’ (period 2011–2016) and ‘after COVID-19’ (late 2021).”

Pre-COVID to post-COVID “changes in menstrual cycle parameters were calculated between those reports.”

The study used “logistic or multinomial logistic regression models,” which are statistical techniques used to analyze relationships between one or more independent variables, to examine whether COVID infection and/or vaccination were related to changes in menstrual cycle characteristics.


The study found that 2,227 vaccinated women, approximately 57.71%, reported either a change in either cycle length or regularity during follow-up, and that COVID-19 vaccination was associated specifically with a “48% higher risk of change to longer cycles in multivariable-adjusted models.”

The study also found that vaccinated women “had a higher risk of increased cycle length than unvaccinated women.”

The authors confirmed their results even after “accounting for pandemic-related stress,” meaning the menstrual changes were not due to psychological influences.

Significantly, the study also found that both messenger RNA (mRNA) vaccines—Pfizer and Moderna—as well as adenovirus-vectored vaccines—Johnson & Johnson and AstraZeneca—were associated with this change.

Moreover, the study found that COVID infection “was not associated with changes in usual menstrual cycle characteristics,” according to the authors.

They emphasize “the importance of monitoring menstrual health in vaccine clinical trials” and recommend “[f]uture work should examine the potential biological mechanisms.”


In their commentary, the Italian researchers who examined these findings referenced another study published March 2022 in Open Medicine that also found around half of reproductive-age women reported menstrual irregularities after getting the first dose of a COVID vaccine, increasing to 60-70% after the second dose.

That study, based on data collected through a web survey that asked women to self-report their symptoms after vaccination, similarly found the type of vaccine did not make a difference.

Read the full study:

Emphasizing the significance of comparing the two studies, the Italian commentary explains that the AJOG paper included a wider range of women, while the Open Medicine paper excluded women with certain medical conditions, in order to make sure the observed changes in menstrual cycles were specifically linked to the COVID-19 vaccine and not to other variables.

The commentary authors write, “Among the main differences between these two survey studies, the one by Wang et al [AJOG] allowed the inclusion of an unselected population, whereas the second one [Open Medicine] excluded women with gynaecological and non-gynaecological diseases, undergoing hormonal and non-hormonal treatments, in perimenopause or menopause, as well as those who had irregular menstrual cycles in the last 12 months before vaccine administration to ensure consistency between the observed outcomes (menstrual irregularities) and the event (COVID-19 vaccination).”


The Italian doctors further revealed that not only do COVID vaccines seem to be linked to menstrual abnormalities, but “other vaccinations, such as the one against human papillomavirus, were found to be associated with short-term menstrual cycle changes” as well.


The commentary authors conclude that the current “available evidence” proves that only COVID-19 vaccination, not COVID-19 infection, is linked to menstrual irregularities.

“[A]ll the available pieces of evidence confirm that only COVID-19 vaccination, but not COVID-19 infection, is associated with menstrual irregularities,” they write.