21st January 2023

COVID-19 vaccines and their effects on the menstrual cycle

New research into the possible impact of the COVID-19 vaccines on women’s menstrual cycles. Here’s what they found out!
COVID-19 vaccines and their effects on the menstrual cycle
Photo: Natracare @Unsplash

What does this new research reveal about the connection between covid-19 vaccines and women’s menstrual cycles?

A new research study has investigated whether the COVID-19 vaccines are associated with changes in women’s menstrual cycles. The research was initiated after a growing number of women expressed concern over their menstrual cycles post-vaccination, with the UK Medicine and Healthcare Products Regulatory Agency (MHRA) receiving more than 36,000 reports of menstrual changes or “unexpected vaginal bleeding following a covid-19 vaccination.”

The reported changes include longer or shorter cycles, missed cycles, heavier or lighter menstrual flow, and intermenstrual spotting.

Although these changes in the menstrual cycle of women may not be considered dangerous to clinicians or scientists, they can become very alarming for women experiencing them. Any changes associated with a woman’s fertility inevitably do generate questions.


The aim of this cohort study was to identify whether there is a link between changes to the menstrual cycle and the COVID-19 vaccines. The study aimed to address the growing concerns over disruptions to menstrual bleeding seen in vaccinated women.

Alison Edelman, M.D., M.P.H., of Oregon Health and Science in Portland was the main investigator in this study, which was funded by the National Institutes of Health.

The investigation was a “prospective cohort study.” This type of data collection method follows a group of individuals over time who are alike in many ways but differ in one characteristic (the study follows a group of women who are vaccinated and those who are unvaccinated). The data was collected between October 1 2020 and  November 7 2021. 

The data collected was from a ‘global cohort’ of users of the menstrual tracking application, ‘Natural Cycles.’ The investigation included 14,936 vaccinated participants and compared them with 4,686 unvaccinated individuals. 32% of the participants were from the United Kingdom, 29% from the US and Canada, and 34% from Europe. The mean age of the participants was 30 years old. The study investigated the results of the Pfizer-BioNTech, the Oxford-AstraZeneca, Moderna, and the Johnson & Johnson COVID-19 vaccine. All participants consented to the ‘use of de-identified data for research.’ 

Participants were encouraged to track their menstrual cycles via the Natural Cycles app. Individuals who had normal pre-vaccination menstrual cycle lengths (average of 24-38 days) were recorded. For the vaccinated cohort the individual’s previous three pre-vaccine cycles and the first covid-19 vaccine dose cycle and subsequent consecutive cycles were recorded. For the unvaccinated cohort ‘four-to-six consecutive cycles from a similar time period’ were recorded.

The cycle after the second vaccine dose was labelled as the ‘post-vaccine cycle’ in order to decipher whether changes to the menstrual cycle are affected by the vaccine.

The investigation adds new research to the field of study and underpins the importance of collecting menstrual cycle data when developing and administering future vaccines.

Are these links legitimate?

The British Medical Journal states that the majority of people who report changes to their menstrual cycle after the vaccination find that it “returns to normal the following cycle.” This suggests that there is no definitive link between the COVID-19 vaccination and significant, long-term changes to a woman’s menstrual cycle.

The Medicine and Healthcare Products Regulatory Agency agrees that no definitive link can be made between variations in menstrual periods and the COVID-19 vaccines because the number of reports (of menstrual changes) is low relative to the number of people who have been vaccinated.

The way in which the data is collected is another cause for concern. When the clinical trials of the covid-19 vaccines were first being undertaken there was no collection of any data relating to the participant’s menstrual cycles and whether they had been affected.  As a result, the link between vaccination and small changes in the menstrual cycle is not clear, and it can not be said for definite that they are even associated with one another.

For example, the study notices cycle changes between the “notional pre-vaccine and post-vaccine cycles in the unvaccinated cohort.” These findings cannot be attributed to the vaccination, and therefore demonstrate how menstrual cycles do tend to change, regardless of the presence of vaccinations, making it difficult to discern the effect of individual factors.

Other studies

Originally, both Norway and the US initiated the first studies on the link between menstrual cycle changes and the COVID-19 vaccines. These two studies both found that the menstrual cycle changes, following the vaccine, are small and short-lived. 

The US study of 3,959 Americans found that the “first dose of the vaccine had no effect on the timing of the subsequent period, while the second dose was associated with a delay of 0.45 days.” The research found that the most affected participants were the ones who had received two doses of the vaccine in the same cycle, “experiencing a 2.32-day delay to their next period.”

Norway’s study reiterated how the vaccine cannot be designated as the reason for menstrual changes indefinitely. The study showed how 37.8% of participants reported “at least one change from normal even in pre-vaccination cycles.”

The results of the new study

The results of the cohort study are very similar to the US-only cohort study. They both suggest evidence of very small cycle length changes after the COVID-19 vaccination. The study showed how the vaccinated cohort had a less than “one-day unadjusted mean increase in their menstrual cycle during the first vaccine dose compared with their three pre-vaccination cycles.” Whereas the unvaccinated cohort “had no significant changes compared with their three cycles.”

The group of people most highly impacted by vaccination was the individuals who received both vaccine doses in one dose, “with an adjusted mean cycle length increase of four days.” However, the research continues to suggest that the vaccine only produces small changes in the menstrual cycle of women and lasts a short amount of time post-vaccination.

Research is still needed to discern why the vaccines would be able to cause any changes in the menstrual cycle. But it is suggested that menstrual cycle changes are caused by the temporary vaccine-related activation of the immune response.

But the changes in menstrual cycles are likely to be the result “of the immune response to vaccination rather than a specific vaccine component.” The hormones that operate the menstrual cycle can be affected by this immune stimulation. But this does not only result from vaccines but also a variety of ‘stimuli.’

Whilst the investigations do not suggest a clear, definitive link between the COVID-19 vaccines and any changes to the menstrual cycle, these clinical trials are invaluable for differentiating between menstrual changes fuelled by ‘medical interventions’ from those that naturally occur within a woman’s body. Reliable and trustworthy information is needed to ease any concerns over menstrual cycle changes.

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