When the COVID-19 vaccines first became available, some people noticed unexpected menstrual bleeding after they received their vaccines. Some trans men were among those affected, which has got to be weird as hell if you thought you were done having periods and then all of a sudden you start menstruating.
Menstrual changes or unexpected bleeding weren’t tracked in the original studies of the COVID vaccines, so nobody was able to say whether these changes were part of a pattern linked to the vaccine. So a team of researchers circulated a survey asking people about their menstruation-related experiences with the vaccine, and now the results have been published. In shorts: If this happened to you, you’re definitely not alone.
How many people had heavier or unexpected bleeding after their vaccine?
Before I tell you the results, there’s one thing that’s important to know: the survey is not based on a representative sample, so we don’t know how common these menstrual side effects were in the population at large. We just know how common they were among people who answered the survey. On the one hand, people who had issues may have been more likely to fill out the survey; but the researchers note there may be reasons why people who had changes in their bleeding or new bleeding might be less likely to fill out the survey. Ultimately, we don’t know.
With that, here are the results:
Of respondents with normal menstrual cycles, 44% said their cycle continued as normal, and 42% said they had heavier bleeding. Of people who don’t typically menstruate, but who used to, the majority of respondents reported breakthrough bleeding. This included 71% of people on long-acting reversible contraceptives such as IUDs, 66% of people who were past menopause, and 39% of people on gender-affirming hormones.
Why did this happen?
We don’t know why it happened, but the authors of the study say that it was premature to tell women last year that there was “no biological mechanism” by which a vaccine could cause breakthrough menstrual bleeding.
But the authors note that changes in menstruation have been reported with other vaccines in the past, including typhoid, hepatitis B, and HPV vaccines. And since the uterus has to switch between bleeding, clotting, and repairing functions, which involve the immune system, something like a vaccine that activates the immune system and causes inflammation may plausibly affect those processes. Senior author Kathryn Clancy told Technology Networks: “When the immune system is activated by something like a vaccine it is going to have all sorts of downstream effects, including on the uterus.”
This doesn’t mean the bleeding is necessarily bad for your health. The authors note that “the uterine reproductive system is flexible and adaptable in the face of stressors to weather short-term challenges in a way that leaves long-term fertility intact.” We already know that stress (both physical and psychological) can have effects on the menstrual cycle.
That said, if you experience bleeding you don’t expect, it may be a good idea to get checked out. Unexpected bleeding when you don’t normally menstruate can occasionally be an early sign of cancer. But it helps to know that it can also be a side effect of a COVID vaccine.
Ultimately, we still don’t know why the heavier and breakthrough bleeding occurred, and we don’t know how common it really was. But at least there is information to say yes, this is a thingand researchers are studying it further.