Answering Your Questions About the Study on COVID-19 Vaccination and Menstruation

And there were lots!

Lots of questions about the study looking at the impact of COVID-19 vaccination on menstrual cycle and bleeding length. Enough over on Instagram that I thought it worthy of a second post.

If you have not read the original post, please do so here. There is also a link to the study in that post.

The aim of the study was to look at changes in the cyclic nature of the menstrual cycle (meaning the time between day 1 of bleeding and the subsequent day 1 of bleeding) post vaccination compared to pre vaccination. Six months of cycle data was collected, 3 cycles pre vaccine and 3 post vaccine (meaning people were vaccinated in their 4th cycle). Data for 6 cycles over the same time frame was collected from unvaccinated people who served as a control group, which is necessary to exclude other causes of changes in the menstrual cycle in the population, for example stress.

The findings showed less than 1 day of difference in menstrual cycle length for the vaccinated group compared with pre vaccine and no changes in the length of bleeding. The only significant change was seen in the subgroup who received 2 doses of the vaccine within one cycle, and they had an average 2 day lengthening of their post vaccination menstrual cycle. In this group 10% of people had a prolongation of their cycle by 8 days or more, versus approximately 5% in the control group.

Okay, onto the questions/comments. These have been collected from my public Instagram feed and also from my Instagram DMs, as well as my public feed on Twitter.

Why Did The Study Just Look at Menstrual Cycle Regularity? What About Spotting? Or the Heaviness of the Flow? Or Period Pain?

There are only a few ways that a medication can impact the menstrual cycle, and one of them is via changes in the signaling from the brain. It is this signaling that recruits the follicles in the ovaries (that contain the immature eggs) and gets them to develop, triggering the production of hormones, ovulation, and then menstrual bleeding. Changes in the cyclic nature of the menstrual cycle would suggest a change in brain signaling.

Another way is via a direct impact on the developing egg, but as fertility and implantation of a fertilized embryo is not affected by the vaccine this seems unlikely.

Because this was an initial study, it was in many ways exploratory. Researchers need a baseline so they can figure out how best to evaluate a concern. As many of the anecdotal reports about menstrual changes included reports of a period that came early or late or a skipped period altogether, looking at the cyclic nature of the menstrual cycle was a very good place to start investigating these claims. By looking at the cyclic nature of the menstrual cycle both pre and post vaccine, researchers were responding to the concerns that had been raised.

It is important to remember that studies typically answer one or two questions. Some outcomes require specific study methods, so not everything we want to know can be crammed together into a single study. I suspect that given the paucity of data about the impact of vaccination in general on the menstrual cycle, the researchers wanted to go for the biggest target that would yield the most useful data for now as well as for potential future studies. Looking at a potential impact on the menstrual cycle achieves that. Evaluating the potential impact of COVID-19 vaccination on the cyclic nature of the menstrual cycle and on the length of bleeding is a study strength, not a weakness.

The researchers address that they did not assess spotting (bleeding between periods). I appreciate that it is frustrating for many people who have reported spotting, but there are multiple causes of spotting that are challenging to analyze with the information provided by the app. Also, it’s hard to sort out spotting when you don’t first know if the cyclic nature of the menstrual cycle has been affected. We do need more data on this topic, and hopefully it will come with time.

As for heaviness of flow, that could not be accurately studied without contacting participants prior to enrollment. Everyone can say they are bleeding or not, meaning the day their cycle started, but the heaviness of menstrual flow is very subjective. To find out if flow is heavy or not you have to define for your research subjects what heavy is and give a specific tool to measure, for example a picture with marks on a pad that indicate light, medium or heavy flow. Basically, this requires set up and this study used data that was already collected. Regardless, it is still reassuring to know the length of flow did not change.

What about pain? Like heaviness of flow, this would require agreeing to a pain scale before starting the study and also more personal medical information.

Why Did The Study Not Include Those Older Than 45?

Menstrual cycle abnormalities over the age of 45 are normal. People will skip periods, sometimes 2 or even 3 or more. I’ve heard from many women over the age of 45 that they got their vaccine and then no period for 3 months. Keep in mind that before the COVID-19 vaccine was introduced I also spoke with women almost daily in my medical practice who were over the age of 45 and had skipped a period for 2 to 3 months. It is not possible to account for that expected randomness in a study period of 6 months.

In addition, when you are first starting out to detect a possible impact of an intervention on the regularity of the menstrual cycle, like the COVID-19 vaccine, you cannot include a group who will, by definition, be more likely to have random cycles as the study progresses. Over a 6 month time frame a 48 year old will be more likely to have irregular cycles in her 6th cycle (i.e. 3 months post vaccine) than in her 1st cycle of the study because she has aged 6 months and menstrual cycle irregularity increases with age in the menopause transition.

Is it possible that down the road we may find a vaccine induced effect for this population? Possibly, I am always willing to consider good science. But given how common menstrual irregularities are over the age of 45, and they are really the norm, this may be a question that proves very challenging to answer. Once we get more data on a younger population who are ovulating, then we may be able to say more.

Why Didn’t They Include People in Menopause?

No one in menopause will be using an app to track their menstrual cycle because they don’t have a cycle. Any bleeding after menopause is not a menstrual period it is abnormal uterine bleeding, and should be investigated as such. I wrote more about that here.

Keep in mind that 11% of people will have abnormal uterine bleeding in the first three years after menopause. So many people who get vaccinated within 3 years of menopause can be expected to have abnormal uterine bleeding unrelated to the vaccine. They were just going to get abnormal bleeding anyway and they coincidentally got vaccinated. So there are lots of opportunities for coincidences.

Could the vaccine trigger abnormal uterine bleeding for those in menopause? It is possible. That is something that needs to be studied, but you can’t blame this study for not addressing it.

What About Those with PCOS or who have Abnormal Cycles? Why were they left out?

To understand if the vaccine impacts people who are not ovulating regularly, you need to first know what happens with ovulatory menstrual cycles. That is why those with irregular cycles were excluded because they were likely not ovulating. So many people with polycystic ovarian syndrome (PCOS) and of course everyone with irregular cycles were excluded from this study. But data from this study will help inform future studies looking at those with irregular cycles.

It is also important to note that there are many different causes of irregular cycles, so to study that specific population you likely need more data than the app in this study collected. For starters, you need a more detailed medical history and likely a physical exam so you can make sure you are not lumping all the different types of irregular cycles into one group.

It Doesn’t Seem Like They Enrolled Enough People.

The researchers had enough numbers to answer the questions they asked. The study included how the researchers came to the conclusion that they would need the number of participants.

A large study is not synonymous with a good study. This is a VERY important point.

Did They Evaluate The Impact for Those on Hormonal Birth Control?

No. The goal was to look at ovulatory cycles.

Does a difference of less than 1 day mean an average? Did some have a period that came 3 days early and another 4 days late, so they canceled each other out?

The researchers compared the difference in cycle length for the post vaccine cycle (cycle 4) to the average of the three cycles before vaccination. They compared people with their own baseline, not to each other. So someone with a 3 day longer period and another person with a 3 day shorter period wouldn’t cancel each other out and produce no change. The person with a 3 day longer cycle was compared with their own baseline cycles before vaccination.

The final result, the less than one day change, is the average of all those individual differences pre and post vaccine changes. But this average is achieved through a much more complex calculation than we think of when we consider determining an average. This required statistics that had to take several variables, some that change, into account.

This chart from the study (see below) shows some of the data, specifically the percentage of people who experienced changes in cycle length after the 1st dose and the 2nd one. Just eyeballing you can see the change in cycle length is pretty evenly distributed between those who were vaccinated and those who were not (the purple is the areas where the two groups match up). For example, among the vaccinated and the unvaccinated approximately the same percentage of people had a cycle that was 3 days longer. With the exception of some outliers (to the right of the graph) with cycles that were delayed by eight days or more (this group turned out to be those who received 2 vaccines in one cycle). It is very reassuring to see such a similar distribution in changes of cycle length between the vaccinated and the unvaccinated.

First Dose Second Dose

I want to restate that changes in cycle length of 7 days or less from menstrual cycle to menstrual cycle are normal. And that all those who experienced the temporary lengthening of their cycle of greater than 8 days after receiving 2 shots in one cycle returned to baseline after 2 cycles.

My Experience Was Different, I Don’t Believe The Study

There are several reasons why your experience may be different, assuming you had regular periods before vaccination.

1. The study may have flaws we don’t yet know about, and hence missed some menstrual changes. For example, how the app collected the data or the researchers may have used the wrong statistical model to account for all the variables. This is why we have peer review and publish studies, so others can look at the data. This is also why we like to see studies replicated. So if we get more data that arrives at different conclusions, then we will have to look at both studies, reanalyze the existing data, and maybe do more studies. That is the meandering path of science.

2. The study may not have evaluated your bleeding concern. For example, there was no data provided on breakthrough bleeding or heavy periods or pain. Changes here could be caused by something other than an impact on ovulation. For example, the immune response in the endometrium. Again, this is why more studies are needed. While frustrating, you can’t fault a study for not answering a question it was never designed to answer.

3. The study may not have reflected the entire population accurately. For example, people who use a menstrual tracking app, or this particular app, may be different in other ways from those who don’t. Again, more studies will help us determine if this played a role or not.

4. The study may be completely accurate and hold up against future studies, and so what you experienced was correlation, meaning your period was always going to be wonky and by chance you received the vaccine right before your wonkiness was due to begin. Remember, in the study approximately 5% of people in the control group (meaning a group made up of entirely unvaccinated people) had a period that was 8 or more days late, but so did 5% of people who were vaccinated. Irregular periods are common and it is human nature to look for patterns when something unusual is happening, especially when it is happening to your body. I get that it can be very hard to unsee what you think is an obvious pattern. When I was pregnant with triplets I had a huge craving for Jamba Juice. I drank them all the time. I had a Jamba Juice the day I ruptured my membranes at 22 ½ weeks, and two days later my first son delivered and died. I stayed pregnant and a few days later I really wanted a Jamba Juice, but my then husband refused to get one for me because he was convinced the Jamba Juice was the cause. It was not, but I could not get him to believe that this was correlation and not cause and effect.

Menstruating People Deserve the Truth!

Of course they do, but no one is hiding the truth. This is the weirdest flex in my direct messages. Like somehow researchers taking the concerns people raised about menstruation and designing a study to address those very concerns is somehow obfuscating the truth?

If You Feel Angry About This Study…

And I’ve had some angry comments and DMs on Instagram since posting about this study.

No one is saying you didn’t have changes to your cycle or that those changes were not awful or scary or that you were not dismissed by your provider. Irregular bleeding, painful periods, heavy periods, and skipped periods are awful and/or scary and it is terrible and unacceptable to be dismissed by your health care provider. What this study tells us is the best data that we have to date suggests that if you received your COVID-19 vaccination spaced over two or more cycles and you had had changes in your period, the vaccine is unlikely the cause. With emphasis on the best data that we have to date, because it may change.

I appreciate that many women have had their menstrual concerns brushed aside for, well, forever. And you are right to be angry about that. But try not to let your legitimate anger that menstrual cycle changes have been neglected in vaccine studies cloud your ability to accept the results of a good study. Isn’t it good to know that the scientific community took your concerns seriously and studied them? And that the initial results are reassuring?

I know this wait and see and back and forth of science can be hard, but as we get more data, we will know more. This data set may even be reanalyzed in future studies. I suspect some researchers will now look at the impact of vaccinating in the earliest part of the cycle.

As more data comes in, what we know may change. And you can be sure that I will report on it. Just don’t yell at me about it on Instagram, okay?