COVID-19 Vaccine and Heavy Periods

A new study details impact, and it's very reassuring

Another study has been published looking at the impact of COVID-19 vaccination on the menstrual cycle, this time looking at the heaviness of menstrual flow.  

Let’s take a look! 

I always get excited when these studies that use menstrual app data come out as they collect large amounts of information prospectively that would otherwise be difficult to accomplish. Not only do they inform us about the potential impact of vaccination, but they also have the opportunity to increase our understanding of the menstrual cycle overall. 

To recap, when the COVID-19 vaccine was rolled out there were reports of menstrual disturbances, including early periods, late periods, skipped periods, heavy periods, and more painful periods. Sadly, and inexcusably, there was almost no data with which to explain these phenomena or to reassure people who were experiencing these issues, because the impact of vaccines on the menstrual cycle was essentially understudied. There was one low-quality study of women who had received the human papillomavirus (HPV) vaccine in Japan suggesting a possible link between the vaccine and heavy and/or irregular bleeding. Unfortunately, given the quality of the study, it couldn’t really tell us anything. And the HPV vaccine isn’t the COVID-19 vaccine.

I was one of the first people to write about potential mechanisms (you can read that early post here). The endometrium is part of the immune system and has cells that are uniquely sensitive to RNA, as it needs to defend against bacteria and viruses that might enter through the vagina. Also, we know some people develop a temporary fever or swollen lymph nodes after vaccination due to activation of the immune system and theoretically this same activation could affect the parts of the brain that control menstruation or impact the endometrium (lining). While many of the health claims I debunk are biologically implausible, in this case it was certainly conceivable that the vaccine could have a temporary impact on menstruation and so I took these reports seriously and eagerly awaited some research, while simultaneously being discouraged that menstruation had essentially been ignored by the vaccine literature, 

Every day I see someone whose periods have become wonky, sometimes for a cycle or two, or sometimes for much longer, and obviously I saw people with these concerns long before anyone had ever heard of COVID-19. As menstrual irregularities are very common, it was crucial for us to have data that could separate the baseline rate of menstrual disturbances from any issues specifically caused by the vaccine. 

What We Learned BEFORE this New Study 

Several groups of researchers have taken the reports of vaccine-related menstrual disturbances seriously. Studies using data collected by menstrual apps (either The Natural Cycles or data from Apple) are perhaps the best, because people using these apps were tracking their menstrual cycles before and after vaccination, so the researchers were able to use data collected in real time, allowing them to compare post vaccine menstrual experience with pre vaccine. Basically, everyone acted as their own control. In total, data from Natural Cycles has looked at over 14,000 vaccinated individuals and more than 4,000 controls and the Apple Watch study had over 8,000 participants who had been vaccinated and more than 1,000 unvaccinated controls. The control group is also important to make sure there wasn’t something affecting everyone’s menstruation — for example stress caused by mass quarantining — which could cause a change that might be mistaken for a vaccine effect. 

Overall, these studies found there was a lengthening of the menstrual cycle post vaccination by less than one day, keeping in mind that a seven-day change in length between cycles (measured from Day 1 to Day 1) is normal. Meaning a change was noted, but one that is not medically concerning. The data from Natural Cycles and Apple were essentially identical, suggesting that the results are likely reliable. 

When researchers looked more closely at the data, they discovered that the changes in the menstrual cycle were found primarily in people who received two vaccine doses in the same menstrual cycle. It seems that about 8% of people who received two doses of the mRNA vaccine in one cycle could expect their next cycle to be delayed by 8 days or more, but it reverted to baseline by the next cycle. It’s unclear if this is because the first dose of the vaccine would have been given very early in the cycle or if it is the cumulative impact of two doses.  

We had less data on the heaviness of the cycle, meaning were people bleeding more after the vaccine or not? The Norwegian Young Adult Cohort, a group of people ages 18-30 already enrolled in a study evaluating the impact of the COVID-19 pandemic, were questioned about the impact of COVID-19 vaccination on the menstrual cycle. At this point most of the people in the study had already received two doses of the vaccine, so the data collected was retrospective, so less reliable. If we take the findings from this study at face value, it suggests that about 7% of people will have heavier bleeding in their cycle post vaccination with an mRNA vaccine and this returns to baseline by the next cycle. Also, among those who experienced heavy bleeding after the first dose, about two-thirds also experienced heavy bleeding after the second dose, suggesting some people might be at greater risk. 

 The New Study

To evaluate changes in the heaviness of flow, researchers once again turned to the Natural Cycles data set. Not everyone collects data on the heaviness of their flow in this app, but there were enough people who did, and ultimately the researchers had data from over 7,000 people who were vaccinated and over 2,000 who weren’t vaccinated as controls. In their analysis they only included people who had a normal cycle in the cycle before vaccination (meaning 24-38 days in length and bleeding for 8 days or less).

One thing this study tells us before we even get to the impact of vaccination is that in any given cycle about 35% of people report a heavier period than typical. This tells us we can’t legitimately evaluate the impact of vaccines on the heaviness of bleeding without a control population. If someone were to just ask people who had been vaccinated about the heaviness of their bleeding and not account for this background rate, it could easily make it appear as if the vaccine were causing heavier bleeding for a third of people! It’s important to note that common doesn’t mean acceptable or should be ignored, but it’s important to know that cycle-to-cycle variations in flow are quite common. 

Using this data researchers looked at the total number of heavy bleeding days in the cycle and the overall heaviness of menstrual flow and compared each person with their own pre vaccination data. They only included people with three recorded menstrual cycles before vaccination and one after vaccination. After controlling for variables, no difference was found in the number of heavy bleeding days, but those who were vaccinated were more likely to report an overall increase in the amount of bleeding. However, the numbers of people affected were small. Among those who were vaccinated 38.4% reported heavier bleeding in their cycle after the vaccine versus 34.5% of those who were unvaccinated, meaning 4% of people had heavier bleeding in their post vaccination cycle likely related to being vaccinated. This was almost identical to what occurred after the second dose, where 3.8% of people who were vaccinated had an increase in menstrual flow. Meaning, if 100 people are vaccinated four will have a heavier flow in their menstrual bleeding post vaccination. And reassuringly, this change resolved by the next cycle. 

It’s important to point out that there are some issues with this study. People in the Natural Cycles cohort on average have a lower BMI and a higher socioeconomic status than the general population, so it is possible that this data set isn’t reflective of the general population. Also, people weren’t obviously given a standardized way to record the heaviness of their flow. Typically, in studies that look at flow all participants are asked to use the same validated scale, so here individuals were compared pre and post vaccine using their own internal scale of menstrual heaviness (as it were). This means the study can’t tell us how significant the increase in flow.

Another issue is the Natural Cycles study didn’t include people ages 45 and up. Menstrual irregularities are so common they are typical during this time, so studying this population will be harder. Is it possible people might be more or less vulnerable to the impact of vaccination during the menopause transition? Yes. The first step will almost certainly be trying to understand why a subset of people with regular menstruation are affected and then proceeding from there. 

The Takeaway

This latest study adds to what we know about the heaviness of flow post vaccine. While the Norwegian Young Adult cohort retrospectively pegged that number at 7%, the Natural Cycles data suggests it may be a little lower at 4%. Both studies tell us the heaviness in flow returns to baseline by the next cycle. I want to be clear, saying this risk of heavy bleeding is small isn’t meant to diminish the impact of heavy bleeding on the person who experiences it. Having unexpected heavy bleeding can be very bothersome and also worrying. Also, knowing heavy bleeding might happen to 4% of people is as important as knowing that 3% experience fever and 5% may have muscle pain post vaccination (those latter two numbers are from the original mRNA COVID-19 vaccine studies).

Hopefully, there will be data forthcoming that looks at hormone levels so we can understand why a small subset of people are affected by the vaccine and also understand the mechanism. What we can take away from these studies is the following:

• Menstrual changes occur post COVID-19 vaccination, and so people should know about the possibility, but they should also know they are uncommon and revert to baseline by the next cycle, which is reassuring.

• People who get two doses of a vaccine in the same menstrual cycle are the ones who are most likely to experience a delay in the onset of their next period.

• About 4-7% of people will have a heavier flow in their cycle post vaccine (the 7% is from retrospective data). 

• If you are in the 4-7% who experiences heavier bleeding in the cycle after you have been vaccinated it seems your risk of experiencing heavier bleeding with a second or booster dose is greater.

• Given the baseline rate of menstrual disturbances and the fact that studies tell us that remembering the last menstrual period can be inaccurate up to 40% of the time, we can’t rely on retrospective data to inform us. People deserve the highest quality data. 

The idea that a permanent change to the menstrual cycle could have happened post vaccine seemed unlikely based on menstrual physiology, but obviously there are things we don’t know. I’m thrilled that researchers took concerns about menstruation seriously, and we now have several studies that show while a small group experience a change, it is temporary. It is reassuring to know that more than 22,000 people and 375,000 menstrual cycles were tracked between the Natural Cycles and the Apple watch study, so we have a lot of data.

How might we practically use this data? Someone who wants to reduce the possibility of heavy bleeding in their first cycle post COVID-19 vaccine/booster might consider ibuprofen 600-800 mg every 8 hours for three days, starting either the day before their expected period or when bleeding starts if they are unsure when their flow might begin, as this can reduce bleeding by 25-35% (you can do this any cycle, by the way). Assuming, of course, they can take non steroidal anti-inflammatory medications (not everyone can). 

And anyone who has experienced long-standing changes in menstruation that temporarily lined up with the vaccine, the data tells us that your provider should be looking for a cause other than vaccination. 

Going forward, hopefully menstrual tracking will be a routine part of vaccine research, as is temperature tracking and checking for swollen lymph nodes. And as more data becomes available, ideally, we will learn more about the mechanisms responsible for the menstrual cycle changes, which may in turn tell us something that we didn’t know about the menstrual cycle. 

References

1. Darney BG, et al. Impact of coronavirus disease 2019 (COVID-19) vaccination on menstrual bleeding quantity: An observational cohort study. BJOG 10 April 2023 https://doi.org/10.1111/1471-0528.17471.

2. Vallvé-Juanico J, et al. The endometrial immune environment of women with endometriosis. Human Reprod Update, 2019;25:565–592.

3. Trogstad, Lill, Increased Occurrence of Menstrual Disturbances in 18- to 30-Year-Old Women after COVID-19 Vaccination (January 1, 2022). Available at SSRN: https://ssrn.com/abstract=3998180 or http://dx.doi.org/10.2139/ssrn.3998180.

4. Edelman A, et al. Association 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.

5. 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.

6. Alvergne A, et al. Effect of COVID-19 vaccination on the timing and flow of menstrual periods in two cohorts. Front. Reprod. Health, 25 July 2022 Sec. Gynecology. Volume 4 - 2022 | https://doi.org/10.3389/frph.2022.952976.