Research published in BMJ Medicine in August 2022 reflected a collaboration between US and UK partners to investigate the effects of COVID-19 vaccination on the menstrual cycle. The study describes how a “range of menstrual cycle changes” had been reported but that clinical trials did not include outcomes related to these.  Led by Professor Alison Edelman of Oregon Health and Science University the study found minimal changes but points to the need for further research. 

The study 

The reported changes to the menstrual cycle following COVID-19 vaccination included longer or shorter cycles, missed cycles, spotting, or a change to the flow. However, the lack of clinical trial data means that until further research took place the only evidence was based on self-report.  

“The absence of prospectively collected menstrual cycle data that include an unvaccinated comparison group limits our ability both to address public concerns about the relation between COVID-19 vaccination and menstrual cycles and to counsel individuals who menstruate about what to expect with vaccination.” 

The study rightly acknowledges that small changes are important to the patient, can “cause alarm”, and, when associated with the vaccine, can “contribute to vaccine hesitancy”. Furthermore, the authors suggest that the lack of evidence about the links between vaccination and menstrual health, “coupled with the long-standing sex specific research inequities” might be interpreted by the public as a “dismissal from the scientific and medical community”. Following global vaccine rollouts, the authors were able to analyse a previous US study and an international sample.  

Methods and results 

The study was a “retrospective cohort analysis of prospectively collected menstrual cycle data” from across a year using a digital fertility awareness application. To participate individuals had to “consent to use of their deidentified data” and report their COVID-19 vaccination status. They then had to record at least 1 cycle after 1st October 2020. Participants ranged from 18-45 years old and contributed a minimum of 4 consecutive cycles. Specific requirements varied for those who received a vaccination and those who didn’t.  

Of 41504 users, 19622 met the inclusion criteria. The final sample included 14936 vaccinated individuals and 4686 unvaccinated individuals. The publication details how the “vaccinated cohort had a less than one day unadjusted mean increase in the length of their menstrual cycle during the first vaccine dose cycle”. By contrast, the unvaccinated cohort had “no significant change in the notional vaccine designated cycle”. After adjustment the difference in change in cycle length by vaccination status was 0.71 days. This suggests that “vaccination is associated with a less than one day adjusted increase in cycle length”. The publication can be accessed online to examine further results in more detail.  

What’s next? 

The results of this study are described as “reassuring” and useful in informing vaccination risk assessments for patients. Despite the observed changes, they are “small compared with normal variation”. However, the authors recommend future research into other changes to the cycle.  

Hopefully these results will be a step towards further investigations into menstrual health but also provide the assurance that many need to take the vaccine. As reported by The Economist in October 2022, these results suggest that there is “probably nothing to worry about”. However, it is important to be sensitive to the fact that the most minute change to a menstrual pattern can be alarming, and without clinical trial data misinformation quickly spreads.  

“False speculation and fake news found fertile ground in an often sensitive and emotive issue”. 

We can only hope that with continued exploration into the effects that vaccination has on menstrual health, this misinformation will be tempered with data. As The Economist identifies, misinformation often comes from misinterpretation. 

“The easiest way to promote a conspiracy theory is to take a grain of truth and blow it out of sensible proportion.” 

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