What we know about COVID vaccines and periods

If you spend any time on social media, you may have witnessed a period tell-all in recent weeks, with women coming online to share changes to their menstrual cycle which they link to getting the COVID-19 vaccine. Dr. Hannah Feiner, a doctor at the Women’s College Hospital Bay Centre for Birth Control in Toronto, has heard these stories too, in her practice. “It’s come up in conversation—maybe their menstrual period is heavier this month, or maybe it’s lighter, or maybe it’s lasted longer or it was shorter,” she says.

So what’s going on? In addition to a sore arm and potentially some flu-like symptoms, can women expect menstrual changes as a side-effect of getting vaccinated against COVID-19? Here’s what we know so far.

Does the COVID-19 vaccine cause menstrual changes?

The short answer is that we just don’t know yet, says Feiner. “There is basically nothing out there, in terms of studies. In fact, menstrual cycle changes tend not to be studied with vaccines to begin with.”

Dr. Chelsea Elwood, reproductive infectious diseases specialist with B.C. Women’s Hospital & Health Centre, agrees. “These conversations are going on but they are anecdotal,” she says.

That said, Feiner says if a patient tells her the vaccine caused a change in their cycle, she tends to believe them. “When a patient experiences something, just because I don’t have a trial to back it up. It doesn’t mean that it’s not happening for that reason,” she says.

In general, a lot of things can cause changes to your menstrual cycle—stress, medications and extreme exercise, like training for a marathon, for example. Also: “A relatively high percentage of women who actually have early miscarriages don’t know about it,” says Feiner. “So they think, ‘My cycle was a week late,’ But it turns out that a lot of those cases the woman was actually pregnant, then had a miscarriage.”

So if a lot of people start paying attention to their cycles after getting the vaccine they may notice changes which may or may not be attributable to the vaccine. “Until people sit down and do their research, and actually include women’s health type questions in vaccine studies, we can’t fully answer that question,” says Elwood.

If it is the vaccine, what could be happening?

Again, because this hasn’t been studied, it’s impossible to say for sure what could be happening in the body to cause a change in menstrual cycles (if anything, at all.) But Dr. Peter Scheufler, Medical Director and Division Head, Women’s Health at Trillium Health Partners in Mississauga, Ont. says it’s possible the inflammatory response from the vaccine could be causing a stress reaction in your body, throwing off your cycle. “That is basically how the vaccine works— it’s creating a barrier against this virus with antibodies and killer T cells. And so when that happens, it’s like a localized stress reaction in the body and stress reactions sometimes throw the cycle off for a little bit,” he says.

He notes that teenagers and women in their forties are more susceptible than other women to having their cycle thrown off due to a stressful event like a vaccine.

So, if the vaccine is causing a stress reaction that’s affecting your period, it will be short-term, says Scheufler. “It’s a transient thing.” By your next cycle, things should be back to normal.

Will this affect my fertility?

The good news is, because changes to your cycle from stress on your body like a vaccine are short term, they won’t cause any long-term effects on your fertility, says Scheufler.

“What happens during menstruation doesn’t really have a whole lot to do with whether or not someone can get pregnant,” adds Feiner. “What’s important is ovulation, and regular menstrual cycles are a good sign that ovulation is occurring.” So if you had a strange period after getting the COVID-19 vaccine but then your cycle went back to normal, there would be no cause for concern, she says.

Elwood believes some people may be confusing the menstrual cycle talk with an earlier, debunked idea that the COVID vaccine can cause infertility or miscarriage because of a similarity between the coronavirus spike protein and a protein in the placenta. (Again, repeat—this idea is not true.) “We have no concerns about that, because it’s not actually grounded in good science,” she says.

Multiple organizations, including the Society of Obstetricians and Gynecologists of Canada, encourage pregnant women to get the vaccine due to the increased risk of hospitalization if you’re pregnant and you get COVID-19, as well as an increased risk of preterm birth.

Why don’t we know more about periods and vaccines?

“There’s a lot of history of exclusion of women’s health issues from vaccine studies and pharmaceutical studies in general,” says Elwood. “This is an opportunity to advocate for women’s health issues in vaccine studies.”

If the data in a large trial showed, for example, that the COVID-19 vaccine could cause changes to your menstrual cycle, doctors could inform their patients of that, and women wouldn’t need to take to social media and do their own online digging to figure out what’s going on. “It’s no different than reporting out fever, sore arm, those kinds of things. It would be good to be able to provide that information for women,” says Elwood.