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Heavy menstrual bleeding and COVID-19 vaccines
The European Medicines Agency recommends listing it as a side effect
The Pharmacovigilance Risk Assessment Committee (PRAC) of the European Medicines Agency just recommended that heavy menstrual bleeding be listed as a side effect of two the mRNA COVID-19 vaccines, Comirnaty (Pfizer-BioNTech) and Spikevax (Moderna). The PRAC said this is based on review of the “available data, including cases reported during clinical trials, cases spontaneously reported in Eudravigilance and findings from the medical literature” there is a “reasonable possibility” that heavy menstrual bleeding is linked to the vaccine.
There isn’t a big new study that has been published, and in fairness, the European Medicines Agency stated that heavy menstrual bleeding post vaccine is of “unknown frequency”. So what data do we have?
The two studies using data from the Natural Cycles menstrual tracking app that I reported on earlier (you can read about them here and here) didn’t report on the heaviness of flow, but they did look at length of bleeding and there was no change. While a longer duration of flow is often a proxy for more blood loss, it’s not definite, meaning these studies don’t really help us here.
Another study that looked at menstrual cycle changes post vaccine included two different cohorts, a small group of women who were followed prospectively through vaccination as well as a retrospective cohort (meaning people who were recruited after their vaccination and then they reported on their cycles). This study did not find any changes in the heaviness of menstrual flow in the people followed prospectively. However, when the researchers looked at the retrospective data (the people who looked back to say what happened to their periods) they found that women using progestin-only contraceptives had a higher rate of heavier bleeding post vaccination. It’s hard to know if these women might be more vulnerable to bleeding issues, as some methods of progestin only contraception are associated with a higher baseline rate of abnormal bleeding, or if they could be reporting expected side effects of their contraception.
There is data on heavy bleeding from the Norwegian Young Adult Cohort. This is a group of people ages 18-30 already enrolled in a study evaluating the impact of the COVID-19 pandemic. The researchers sent an electronic questionnaire to this cohort in October 2021 asking 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. What they found was that 7.6% of respondents reported their menstrual cycle pre vaccine was heavier than expected and 13.6% reported a heavier period after the first dose of the vaccine. Regarding the second dose of the mRNA vaccine, the prevalence of heavy bleeding was 8.2% before and 15.3% after the second dose. 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 heavy bleeding returned to baseline by two months after the first dose of the vaccine. Basically, this study 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.
A web-based survey from the United Kingdom that started enrolling in March 2021 also evaluated the impact of COVID-19 vaccination on the menstrual cycle. The investigators tried to advertise their study in a neutral way to avoid disproportionately recruiting people with bleeding concerns, important to limit bias. In this study 80% of people reported no change to their menstrual cycle, with 6.1% reporting more disruption, 1.5% reporting less disruption and 11.5% reporting what the researchers lumped together as “Other changes”, which, could mean a change in cycle length, regularity, length or bleeding, heaviness of flow, or PMS. In this study, people taking an estrogen containing contraception were less likely to report changes and those who smoked and who had previously had an infection with COVID-19 were more likely to report menstrual disturbances. This study doesn’t tell us the rate of heavy bleeding per se, it tells us that 80% had no changes in bleeding and the most common words used to describe changes by those who reported them were “cramps”, “late”, “early”, “spotting”, “heavy” and “irregular”. Interestingly, there is data suggesting the side effects from COVID-19 vaccination overall are more common among people who had previously had an infection with COVID-19, so that would support a previous COVID-19 infection being linked with a greater chance of menstrual side effects.
Another web-based survey from the United States reported that 42% of people with regular menstrual cycles had heavier bleeding than usual after the vaccine. People with menstrual cycle changes may be over sampled in this study, as the investigators pointed out that “many participants learned of the survey after performing an online search to investigate their own menstrual experiences and finding social media and/or news coverage of this project.” Some of the factors associated with a heavier flow in this study are factors known to be associated with a heavier flow in general, such as being older, having previously been pregnant, and having been diagnosed with a reproductive health condition. Unfortunately, this study can’t tell us if these people were more likely to have a vaccine-related change in menstruation because of a biological vulnerability (meaning these conditions increased their risk of having bleeding complications) or if perhaps this bleeding change was related to these other conditions, meaning correlation not causation. Interestingly, hormonal contraception was also associated with a heavier flow post vaccination.
It’s important to point out that bleeding issues are very common. For example, in the Norwegian study about 37.8% of people reported a menstrual irregularity in the month prior to vaccination. We also know some people are at greater risk of developing bleeding abnormalities, including a heavy flow. So getting good data here is important so we can sort out what bleeding issues might have happened regardless of being vaccinated and what is causally related to the vaccine.
So where do we stand on heavy bleeding? We have a small prospective study that showed no change in flow, but it was probably too small to pick up the change in flow identified in the Norwegian study. The second best data is from the Norwegian study which indicates about 7% of people have a heavier flow in the cycle post vaccination and that returns to baseline after the vaccination series is finished. This aligns with a temporary immune system activation. Meaning, some people may develop a temporary fever or muscle pain post vaccination and others may have an increase in menstrual flow.
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. Knowing heavy bleeding might happen to 7% of people is as important as knowing fever might happen to 3% of people or muscle pain to 5% (I pulled those from one of the original COVID-19 vaccine studies).
It’s interesting that the European Medicines Agency issued this recommendation now as there is no new publicly available data or recently published research. It’s possible they have data from vaccine companies or from Eudravigilence (basically like VAERS in the US) not yet available to the public. It’s also possible they consider what is available to be enough to make the recommendation, after all the language they use is “reasonable possibility”. It is curious they didn’t mention the delay in menstruation (the slight prolonging of the cycle), as this has been shown in the three prospective studies linked above as well as from data collected prospectively from Apple watch users in the Apple Women’s Health Study.
For now, it seems reasonable to assume the Norwegian data is the best on heaviness of flow, and so it seems appropriate tell people that overall there is a 7% chance they will have a heavy flow in the cycle after mRNA vaccination and those who experience that chance are more likely to also experience it after their second dose. The good news is that like other unpleasant vaccine-related side effects, such as fever and swollen lymph nodes, the effect is temporary.
It’s important to have this data on menstruation so people aren’t surprised with side effects. Just like some people plan to take the day off post vaccination if they are prone to fevers or feeling unwell, knowing your next period might be heavier can allow people to take precautions and/or plan to make the experience less bothersome.
I’d like to add that all of this is retrospective data, so it’s always possible that what we think we know will change with more studies. It goes without saying that we need more research into vaccine-related menstrual changes. And of course, as that research becomes available I will be reporting on it here.