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Home » COVID vaccines slightly affect menstrual cycles but do not affect menses

COVID vaccines slightly affect menstrual cycles but do not affect menses

A new peer-reviewed paper showed that COVID-19 vaccines slightly and temporarily increased the length of menstrual cycles. The research also showed that the vaccine did not change the number of days of menses. The effects are so minor as to not warrant concerns about these vaccines.

This brief post is just going to lay out the data from the article and try to show that if you are a woman considering the vaccine, this shouldn’t be a concern.

COVID-19 vaccines menstrual cycles
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Paper about COVID-19 vaccines and menstrual cycle

In a study published in Obstetrics & Gynecology on 5 January 2022, Alison Edelman, M.D., M.P.H., of Oregon Health & Science University, Portland, and colleagues studied 3,959 individuals (vaccinated 2,403; unvaccinated 1,556). The vaccinated cohort received the Pfizer (55%), Moderna (35%), or Johnson & Johnson (7%) vaccines.

The study included U.S. residents aged 18–45 years with normal cycle lengths (24–38 days) for three consecutive cycles before the first vaccine dose followed by vaccine-dose cycles (cycles 4–6) or, if unvaccinated, six cycles over a similar time period.

The researchers then calculated the mean within-individual change in cycle and menses length (three prevaccine cycles vs first- and second-dose cycles in the vaccinated cohort, and the first three cycles vs cycles four and five in the unvaccinated cohort).

The results:

  • The study had more than 99% power to detect 1-day difference in cycle length or a 0.5 day change in menses length by vaccine status
  • COVID-19 vaccines were associated with a less than 0.71-day increase in menstrual cycles after one vaccine dose, and a 0.91-day increase after the second dose.
  • Unvaccinated individuals had no significant change in the cycle length.
  • There was no change in menses length associated with vaccination status.
  • The change appeared to decrease in subsequen cycles after vaccines, indicating that the menstrual changes are temporary.

This study was funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) and the National Institute of Health’s Office of Research on Women’s Health funded the study, which was part of $1.67 million awarded to five institutions to explore potential links between COVID-19 vaccination and menstrual changes. If you don’t think scientific institutions don’t spend money and effort to examine issues with vaccines, including the COVID-19 vaccines and menstrual cycles, this might convince you otherwise.

This study had some important limitations:

  1. The study may not be applical to the US or other populations as a whole because the population included in this study were more likely to be white, college-educated, not using hormonal conraception, and have lower BMIs than a random cohort of women.
  2. The study only included women with consistent average cycle lengths. We do not know what the COVID-19 vaccines effect on menstrual cycles and menses length on women with inconsistent average cycle lengths.
  3. The length of cycles and menses were self-reported, so there may be unintentional bias introduced into the data set.
  4. The study was not set up to determine causality — it is entirely possible there are confounders, such as the demographics of the cohort, may have introduced bias into the results. Furthermore, the study did not provide a biologically plausible mechanism, critical for establishing causality, that might link the COVID-19 vaccines to length of menstrual cycles and menses.
  5. The study was not long enough to determine if there were further changes in menstrual cycles or menses past a few months. I do not expect that there would be, as vaccine effects are always short-term, but that data would be important to counteract misuse of this study by anti-vaccine activists.
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The study authors concluded:

Our findings are reassuring; we find no population-level clinically meaningful change in menstrual cycle length associated with COVID19 vaccination. Our findings support and help explain the self-reports of changes in cycle length. Individuals receiving two COVID-19 vaccine doses in a single cycle do appear to experience a longer but temporary cycle length change. Coronavirus disease 2019 (COVID-19) vaccination is not associated with changes in menses length. Questions remain about other possible changes in menstrual cycles, such as menstrual symptoms, unscheduled bleeding, and changes in the quality and quantity of menstrual bleeding.

There are several other studies sponsored by the NICHD which will provide us with more data going forward.

However, these data do not indicate a serious safety signal with the COVID-19 vaccines, and women, including ones who are pregnant, should not use this study as a reason to avoid the vaccines, since the changes were small and temporary.


Michael Simpson

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