New study provides even more info on the impact of COVID-19 vaccination on the menstrual cycle

Most people have minimal to no impact

A new study provides us with even more information about the impact of COVID-19 vaccination on the menstrual cycle, and it’s very reassuring, although you might not know that based on some of the headlines. Sigh.

This study is an even bigger look at the data set collected by the menstrual tracking app, Natural Cycles. The first study, published in January and included cycle information from over 2,400 people who were vaccinated, found the percentage of people who experienced an 8 day or more difference in cycle length (day 1 of bleeding to day 1 of bleeding) was 5.2% for those who were vaccinated compared with 4.3% for those who were unvaccinated. Most of the cycle changes were driven by a small subset of people who received both doses of the vaccine in the same menstrual cycle. For these people, 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. However, this quickly returned to baseline.

Just so we’re all on the same page, a variation in menstrual cycle length of up to seven days per cycle is considered to be typical. And the initial study didn’t inform us about the amount of bleeding.

With this new study, researchers once again abstracted data collected from the Natural Cycles app, meaning the menstrual cycle information was collected in real time before and after vaccination. This approach is the next best thing to an actual study where people are enrolled and tracked. There are some surveys that have been published looking at what people have reported regarding their menstrual cycle after vaccination, but that data isn’t as good since people who have concerns about bleeding would be more likely to sign up for the study, which introduces a bias.

I can’t stress enough how important it is that there are several pre vaccine cycles for this kind of research. This is because menstrual cycles normally vary up to 7 days cycle to cycle, and in addition there are non-vaccine related phenomena that can affect menstrual cycles. Researchers need to be able to compared what happened post vaccination with several pre vaccination cycles.

This new study captures data from over 19,000 people in the United States, United Kingdom, Canada and Europe and includes over 14,000 vaccinated individuals and more than 4,0000 unvaccinated individuals who acted as controls. More than 250,000 menstrual cycles were evaluated. There was a minimum of four cycles of data collected, three pre vaccination and at least one post vaccination and for those who were unvaccinated, four to six cycles over the same time frame were chosen with the last cycle compared with the previous ones. Each patient served as their own control, meaning the change in menstrual cycle length (day 1 to day 1) post vaccination or for the fourth cycle for the unvaccinated was compared to each person’s previous cycles.

The results?

Overall, vaccination was associated with an increase in the length of the menstrual cycle of 0.71 days. While some people may notice this change, less than one day change in menstrual cycle length isn’t medically concerning. In addition, cycles returned to their pre vaccination baseline by the next cycle.

Like the previous study published by this group, this small difference in menstrual cycle length overall was largely driven by the people who received two doses in one cycle. For this group, the first post vaccination cycle was an average of 3.7 days longer. Let’s look at the results where the two doses per cycle group are removed and compared against the first and the second dose.

What about the percentage of people who had a medically notable change in their cycle, meaning a change of 8 or more days in cycle length? Looking at all participants, this was 6.2% for the vaccinated group and 5% for the unvaccinated group. If we just look at the people who received two doses in one cycle, 13.5% had an increase in cycle length of eight days or more, versus 5% for the unvaccinated control group. In the previous study from January, 10.6% of people who received two doses in one cycle had an increase in menstrual cycle length of eight or more days, so it seems there is more noticeable effect for a minority of people who get two doses of the COVID-19 vaccination in one cycle. When the group with two doses per cycle were removed from the analysis, receiving a single vaccination was not associated with an increase in cycle length of eight or more days.

What about the bleeding?

There was no difference detected in the length of bleeding. While this doesn’t provide data on the heaviness of menstrual periods, in general, cycle length correlates with the amount of bleeding, so it’s reassuring. We do need more data here, but this will require more than a menstrual tracking app can deliver. When researchers want to study the heaviness of menstrual flow, participants have to be given a scale to use in advance. The same goes for studying period pain. Basically, my definition of heavy periods or painful periods may not be your definition, which doesn’t work well in research.

So what’s going on here medically speaking?

Why might two doses of the vaccine in the same cycle prolong that menstrual cycle? For me, the best guess is the first dose is given at the very beginning of the cycle (necessary to allow time for the second dose before the next period) and the immune response here could have an effect on how the follicles start to develop, basically delaying then for a day or two (either due to an effect on hormones or on the follicles themselves). It’s unlikely that the second dose, which is given closer to the end of the cycle, would have an impact as once ovulation has occurred menstrual timing is harder to alter. For the second dose to prolong menstruation it would have to extend the life of the corpus luteum, and based one what we know, that doesn’t seem biologically plausible. I think it’s unlikely that the menstrual effect of two doses is related to an effect on the endometrium, because that would be more likely to cause spotting or an earlier cycle. Hopefully as more studies become available we can better understand this mechanism.

What’s reassuring is that when the menstrual cycle is prolonged, it’s temporary and returns to baseline by the next cycle after vaccination, and so this effect can be thought of as something like lymph node swelling or a fever after vaccination. A temporary immune response experienced by a small percentage of people. Regardless of the mechanism(s), we know that fertility post vaccination isn’t impaired.

What about people who report that their periods have been irregular for months post vaccination? They may not have been paying as close attention to their cycles pre vaccination, and so their periods post vaccination were indeed irregular, but they were irregular to the same extent before the vaccination. Another possibility is the change in their menstrual cycle is new, but due to another cause. Remember, 5% of people in the study who were not vaccinated had a change in their menstrual cycle length of 8 or more days. Given the millions of people getting vaccinated, this will add up to a lot of people.

More data is definitely needed going forward. It’s important to understand the mechanism of even minor changes as this can tell us more about the menstrual cycle and of course, we need to know if there are other implications. For example, is the antibody response (meaning protection) from the vaccine as robust when two doses are given in one cycle, more robust, or is there no change? We also need quality data on the heaviness of bleeding and the impact on menstrual pain.

Why have we not heard reports about other vaccines and menstrual cycle length?

It’s unfortunately not previously been studied, and this is a huge oversight. Hopefully, this will be rectified going forward.

More people are tracking menstrual cycles than ever before, so there is a greater awareness of even subtle changes.

If two doses of a vaccine are needed to produce an effect, the COVID-19 vaccine is atypical in that there is a decent chance two doses can be given in one menstrual cycle. Most other adult vaccinations are a single dose, or the second dose is spaced months out. For example, influenza is one dose. The second dose of the HPV vaccine for those who are 15 years or older is at 1-2 months, so for most people the second dose will fall in the subsequent menstrual cycle. The shingles vaccine is typically given to those who are 50 and older and the second dose is at least two months after the first one.

Previously, side effects from other vaccines haven’t been amplified for political gain in this way. This doesn’t mean people weren’t experiencing cycle changes, but awful people like Dr. Christiane Northrup amplified stories in a twisted, untruthful way.

The menstrual cycle needs to be part of all vaccine research. We simply don’t know what we don’t know. To get good data this will require people enrolling in a vaccine study several months before they receive the vaccine so their pre vaccine cycles are available for comparison and so they can be instructed how to record pain scores and amount of bleeding. This doesn’t mean it’s impossible to do, but the more that’s asked of participants, the harder it can be to get enough people to enroll and/or complete the study and of course the greater the expense. Then again, hopefully raising awareness about this important need will encourage more people to enroll in these kinds of studies and for regulators to insist that the menstrual cycle be included in all vaccine studies going forward.

References

Edelman A et al. Association between menstrual cycle length and covid-19 vaccination: global, retrospective cohort study of prospectively collected data, BMJMED 2022:1:e000297

New study details impact of COVID-19 vaccination on the menstrual cycle. Dr. Jen Gunter The Vajenda accessed October 1, 2022