Study finds slightly longer menstrual cycle after COVID vaccine

Woman checking smartphone
Woman checking smartphone

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COVID-19 vaccines are associated with a temporary average increase in menstrual cycle length of less than 1 day but no change in period length—confirming previous study findings, according to an international study published yesterday in BMJ Medicine.

A team led by Oregon Health & Science University researchers analyzed data from 19,622 users of the Natural Cycles fertility-tracking app who had menstrual cycles of 24 to 38 days and were aged 18 to 45 years from Oct 1, 2020, to Nov 7, 2021. Menstrual cycles are measured from the first day of one period to the first day of the next.

Most participants were younger than 35 years and were from the United Kingdom (31.7%), the United States and Canada (28.6%), and Europe (33.6%).

Determining prevalence, clear link

A total of 14,936 women were vaccinated and had consecutive data on at least three menstrual cycles before, and one after, the COVID-19 pandemic began, while 4,686 were unvaccinated and had data on at least four consecutive cycles over the same time span.

Two thirds (66.5%) of the vaccinated women had received the Pfizer/BioNTech COVID-19 vaccine, 17.5% received Moderna, 9.1% received AstraZeneca/Oxford, 1.9% were given Johnson & Johnson (J&J), and the remainder received the Covishield, Sputnik, Covaxin, Sinopharm, or Sinovac versions. The Pfizer, Moderna, and J&J vaccines are authorized for use in the United States.

The researchers measured the mean change in menstrual cycle length in vaccinated participants by averaging the three cycles before vaccination with the two after the first and second doses of COVID-19 vaccine and then used models to estimate the adjusted change in cycle length between the vaccinated and unvaccinated groups.

The authors noted that the US Vaccine Adverse Event Reporting System and the UK Medicines and Healthcare Products Regulatory Agency's Yellow Card surveillance program have received reports of menstrual changes after COVID-19 vaccination. "But, as passive systems relying on self-report, the findings are useful for identifying potential issues but are unable to determine prevalence or a clear association," they wrote.

No change in length of bleeding

Participants' first and second menstrual cycles after vaccination were longer by less than a day than those of unvaccinated women (0.71 day after the first dose and 0.56 day after the second). The adjusted difference in cycle length was larger in recipients of two doses during a single cycle than in those who received only one dose (3.70 days longer). The length of periods was unchanged.

While single-dose-per-cycle recipients saw a similar cycle length a month after vaccination as they did before immunization (0.02-day change), two-dose-per-cycle recipients experienced an increase of 0.85 day relative to the unvaccinated group. Cycle length didn't differ by vaccine type, whether mRNA, adenovirus vector, or inactivated virus.

A total of 6.2% of vaccinated participants and 5.0% of unvaccinated women reported an increase in cycle length of 8 or more days. Younger women and those with a longer prevaccination cycle length were most likely to report this increase.

Changes within normal limit

A news release from the National Institutes of Health, the study's funder, notes that a change in menstrual cycle length of less than 8 days is considered normal but that women may conflate cycle changes after vaccination with lower infertility, possibly fueling vaccine hesitancy.

"These findings provide additional information for counseling women on what to expect after vaccination," Diana Bianchi, MD, director of the National Institute of Child Health and Human Development, who wasn't involved in the study, said in the release. "Changes following vaccination appear to be small, within the normal range of variation, and temporary."

The results are similar to those of a US study led by the same authors and published in Obstetrics & Gynecology in April 2022. While the increase in cycle length is considered within normal range, they said in the new study, "any change, even if small and not clinically relevant, is important to the public, and even more so in the context of a new vaccine."

"The absence of evidence about vaccines and menstrual health coupled with the long standing sex specific research inequities can also be interpreted by the public as a dismissal from the scientific and medical community," they added.

The team called for future studies on aspects of vaccination-related menstrual-cycle changes such as unanticipated vaginal bleeding and pain with menstruation and for research into the mechanisms behind such changes.

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