New study details impact of COVID-19 vaccine on menstrual cycle

It is very reassuring

A new study is out about the potential impact of COVID-19 vaccine on menstruation, and it is the kind of data we have been waiting for!

There have been lots of anecdotal reports on social media about a potential impact of the COVID-19 vaccine on menstruation. As the clinical trials evaluating the vaccine did not record menstrual changes as a side effect, turning to those studies for answers wasn’t possible. While there are several potential mechanisms to explain menstrual irregularities post COVID-19 vaccination (something I have addressed previously in detail in this post), it is also important to note that menstrual abnormalities are fairly common and so sorting out correlation from causation simply wasn’t possible. 

But now we have some quality data!

Researchers looked at data on cycles collected by individuals using the Natural Cycles app. This was an ideal set up to look at menstrual data because it was possible to find people who had been tracking their menstrual cycle before they were vaccinated. This is key, because to understand if the menstrual cycle has changed, a baseline evaluation of the menstrual cycle is needed. The researchers could then look back post vaccination and compare with the pre vaccination menstrual cycles.

The eligibility requirements for the study were as follows:

• 18–45 years

• Normal cycle lengths (which is 24–38 days). 

• Recorded 6 consecutive cycles, and for those who were vaccinated 3 cycles before and 3 cycles after vaccination were required. 

• Not using hormonal contraception.

The study included 3,959 individuals, 2,403 were vaccinated and 1,556 unvaccinated and among those who were vaccinated 55% received Pfizer, 35% Moderna, and 7% Johnson & Johnson. Unvaccinated individuals were included to ensure there was no baseline change in menstrual length due to pandemic-related stress or other unknown reason. 

One goal was to compare menstrual cycle length in the cycle immediately post vaccination with the average of the three pre vaccination cycles, where menstrual cycle length is day 1 of bleeding to day 1 of bleeding in the next cycle. Another outcome was any change in the length of menses (the number of days of bleeding) pre versus post vaccination. This assessment was done for the first dose of the vaccine and again for the second (when applicable). Those who were not vaccinated had their fourth menstrual cycle compared with the average of their first three cycles. The researchers also looked at anyone who had a change in eight days or more of cycle length, meaning a cycle that is 8 or more days longer or shorter than expected. A difference of eight days is considered medically significant. 

And the findings?

There was no change in the menstrual cycle over the six cycles for the unvaccinated individuals, meaning there was no baseline pandemic-related (or other cause) of menstrual disturbances from December 2020 to July 2021, the time frame used to collect the data.  

For the first vaccination there was an average increase in menstrual cycle length of less than one day compared with the average of the three pre vaccination cycles. This is not considered medically significant. The proportion of people who experienced a significant change in cycle length, which is defined as 8 days or more, was 4.3% for unvaccinated vs 5.2% for vaccinated, which was not significant statistically.  

After the second vaccination there was also less than a 1 day difference in menstrual cycle length compared with pre vaccination. However, more people reported a change in cycle length of 8 days or more, 4.6% for the unvaccinated vs 6.5% vaccinated.

According to the researchers, most of the cycle changes were driven by a small subset of people who received both vaccinations in the same menstrual cycle. Among those who received two doses in one cycle there was an average 2-day increase in cycle-length and 10.6% had an increase in their menstrual cycle of 8 days or more. By the 6th cycle, cycle-length had returned to baseline for all those who received 2 doses in one cycle.

It is certainly possible that two doses in one cycle could have a more profound effect, either because the first dose would have to been given very early in the cycle to get in the two doses (as the vaccines are dosed three to four weeks apart) and the start of the cycle may provide the best opportunity for the vaccine to temporarily impact the messaging from the brain that starts the development of the eggs. An impact here could theoretically lengthen the cycle. Another possibility is two doses of the vaccine in one cycle provided a cumulative effect. Regardless of the mechanism, it was temporary and cycle length was restored within the next two cycles.  

There was no change in length of bleeding after the first or the second dose. 

What is the take away?

The idea that the menstrual cycle keeps time like an atomic clock is simply not true. A variation in menstrual cycle length of less than 8 days cycle to cycle is normal. On top of that, variations of 8 days or longer happen more often than people think, as evidenced by approximately 5% of people who were not vaccinated having variations in their menstrual cycle length of 8 days or more each cycle. 

This study didn’t provide data on the amount of blood flow, changes in pain, or spotting (mid cycle bleeding), so hopefully more data will be forthcoming. But overall it is very reassuring that vaccination appears to have a minimal effect on the length of menstrual cycles. For a small percentage of people who receive two doses of the vaccine in one cycle there may be a delay in the next cycle of eight days or more, but the cycle returns to baseline over the next two cycles. 

I would not delay a first or second dose of the COVID-19 vaccine to try to spread the vaccine over two menstrual cycles. The chance of a cycle related issue is still low and is only temporary. The far greater risk to the menstrual cycle is from contracting COVID-19. 

Hopefully, many will find this data reassuring and the information on the subgroup of those receiving two doses and being more likely to have a delay in their next menstrual cycle will help inform future studies. 

References

Alison Edelman, MD, MPH, Emily R. Boniface, MPH, Eleonora Benhar, PhD, Leo Han, MD, MPH, Kristen A. Matteson, MD, MPH, Carlotta Favaro, PhD, Jack T. Pearson, PhD, and Blair G. Darney, PhD, MPHAssociation Between Menstrual Cycle Length and Coronavirus Disease 2019 (COVID-19) Vaccination. A U.S. Cohort. Obstet Gynecol 2022 https://journals.lww.com/greenjournal/Fulltext/9900/Association_Between_Menstrual_Cycle_Length_and.357.aspx