As more and more Americans receive their first or second doses of the Pfizer or Moderna COVID vaccines every single day, some people who menstruate are reporting changes to their periods after getting vaccinated.
Dr. Kate Clancy, an associate professor of anthropology at the University of Illinois, posted her experience on Twitter in February and received hundreds of reports from those experiencing what she pondered could be a little-publicized response to the two available mRNA vaccines.
"A colleague told me she has heard from others that their periods were heavy post-vax. I'm curious whether other menstruators have noticed changes too? I'm a week and a half out from dose 1 of Moderna, got my period maybe a day or so early, and am gushing like I'm in my 20s again," Clancy wrote. "Does this have to do with the way the vax response is mounting a broader inflammatory response, possibly moreso because of the lipid nanoparticle or mRNA mechanism? Either way I am fascinated!"
Several people responded to her tweet noting irregularities in their cycles. One woman in menopause said she was experiencing her first period in 28 months, another said hers was "bad enough" that her doctor made her get bloodwork done, while a third said she started her period in the middle of a birth control pack - something that has never happened to her in 12 years of taking the pill.
Several more reported their periods arrived earlier and the flow was much heavier than usual, while others still reported changes to their menstruation cycle not after getting vaccinated but after contracting COVID-19 itself.
The responses varied widely and were so numerous, Clancy said she and her colleague would create a tool to try to gather respondents' different experiences. She then tweeted a link to the ensuing voluntary research survey on April 7, calling out on social media, "vaccinated menstruators assemble."
"Side effects are a common and even important element of the vaccine response, and bleeding patterns can be an important way to understand how our immune systems are activated," the survey reads, noting it asks about timing of vaccines, menstrual cycles, reproductive history and general health, taking about 15 to 20 minutes.
Clancy has since taken to social media multiple times to clear up misconceptions about her survey and what she's studying as it relates to the vaccines and how they may impact menstruation.
Earlier this month, she replied to someone saying there was something "very wrong" with a vaccine that may prompt those in menopause to have a period by saying, "I gladly took the vaccine & hope others take it. Our research is to bear witness, look at associations, understand mechanisms."
"Just as we deserve to know if we might get a sore arm, fever, or fatigue after the vaccine, menstruators deserve to know if there is any likelihood of menstrual changes. This is especially true for folks with endo, menorrhagia, etc.," Clancy added. "If we find associations between certain menstrual experiences and timing of the vaccine, health history, etc, that could help clinicians advise menstruators on how to avoid or handle adverse effects."
Health experts have noted that menstrual changes have been documented in recent months outside of vaccinations as well.
A study published in January found that 50 of 177 patients, about 28%, with COVID-19 and menstrual records reported changes to their cycles after contracting the coronavirus, while 25% reported changes to the volume of their menstruation. Some replying to Clancy's initial tweet reported they too had seen changes to their periods after contracting COVID.
Even without contracting COVID or getting vaccinated, menstrual changes have been reported possibly stemming from the overall pandemic environment itself. A Washington Post report from August found that several gynecologists "confirmed that many of their patients are reporting skipped periods or have noticed increases or decreases in cycle length, blood volume and level of menstrual-related pain."
One of those doctors noted that "any significant illness can throw the menstrual system out of rhythm," while several cited stress as a factor.
When it comes to the vaccines, an M.D.-Ph.D. student and writer in residence at the Yale School of Medicine together penned an opinion piece for the New York Times on Tuesday entitled "No, We Don’t Know if Vaccines Change Your Period."
"So far, there’s no data linking the vaccines to changes in menstruation," they wrote. "Even if there is a connection, one unusual period is no cause for alarm. There is a long list of triggers that can cause changes to the menstrual cycle, including stress, illness and changes in diet and physical activity."
But Clancy said last week that it was "really bothering" her to hear doctors say there was "no link" between the vaccines and menstrual changes, or that it's "just stress."
"I think this is coming from a good place - that MDs do not want to concern people nor add to the vaccine hesitancy that some are feeling. However in the long run this will have the opposite effect. Gaslighting patients and saying their experiences aren't real is paternalistic," she tweeted. "I wish more MDs respected our expertise & realized maybe we are capturing something that is, in fact, quite real. And that seeking to understand a side effect does not mean we say that side effect is worrisome, dangerous, or not typical of a vaccine immune response."
Clancy also previously noted that a decision to get a vaccine is about weighing the pros and cons.
"The existence of side effects to any treatment is not necessarily a reason to stop that treatment - see how much hormonal contraceptives suck as just one example," she wrote. "It's about costs v benefits. With the covid vaccine not taking it can mean permanent illness or death."
"So I'll take the heavy period or two, and move on with my life with far less anxiety about the pandemic and far less risk of death. That doesn't mean MDs should keep discounting the existence of this side effect or make menstruators feel like it's not important to address," she noted, adding, "People with a history of heavy menstrual bleeding or any bleeding disorder need to talk to their doctor about which vaccine might be best for them. The research we are doing on this issue will undoubtedly help those people, and bring comfort and information to other menstruators."
Chicago Department of Public Health Commissioner Dr. Allison Arwady was asked about the possibility of vaccinations impacting menstruation in a Facebook live broadcast on Thursday and again highlighted the safety of the vaccines.
"Two hundred and twenty million Americans have gotten a first dose of vaccine, right? So, among 220 million Americans, there are people who will have, you know a herpes outbreak, who will have a changed menstruation cycle, etc.," she said. "What has been interesting, I think, is that, that this has really raised some awareness for wanting to make sure that we're asking questions about things like changes in menstruation, right? Most vaccine trials, most trials in general actually, unless they're really focused on women's reproductive health, may not even ask questions like that and that perhaps points to some biases in terms of how, you know, trials in general for medications, etc. are set up."
"I have not seen anything, to be very clear, that suggests that there is any concerning side effects in the way that would last, I know and there's a local researcher who is looking at some of this related menstruation, but very clear there's not been any link to, you know, problems with fertility, you know, anything that's long-lasting but, you know, the goal of getting a vaccine is for your immune system to learn how to protect yourself against COVID and your immune system can interact, can interface with your, you know, your hormonal levels, etc. and so there is at least some biological plausibility that you could have, you know, some change in terms of a heavier period or a lighter period for example right after getting the vaccination," Arwady continued.
"So I'm glad that people are looking at it but I don't want people to feel like there's something dangerous about that," she said. "We've not detected anything concerning safety concern for any of these vaccines, but I think it's highlighting just that we don't always ask these kinds of questions sometimes in the trials and I think that's a good thing to highlight."
Earlier this month, Arwady noted in a previous Facebook Live that women are more likely than men to report side effects from the vaccine.
"Some of this is because women may just be better reporters… but there probably is something real to this too because something else interesting for those who may not know as much about immunity is that autoimmune diseases? Much, more likely in women, too," Arwady said on April 8. "And even the, like, more serious like the allergic reactions, the more serious allergic reactions? More likely in women."
Arwady said estrogen can elevate immune responses, while testosterone can decrease it. At the same time, she noted that "a lot of your immune modulating genes" can live on an "x" chromosome, which women have two of, while men have one.
"So there's all these reasons that sort of immunity in general goes up a little bit different in women than it does in men," she said. "And so we're seeing women, a little more likely to report some of the side effects."
Data from the CDC also reported women were more likely to experience side effects than men, according monitoring from the first month of vaccinations.
From Dec. 14 through Jan. 13, more than 79% of side effects were reported by women, who received roughly 61.2% of the doses given, the data showed.
U.S. regulators last week recommended a nationwide pause on administering the single-shot Johnson & Johnson COVID vaccine amid an investigation into reports of six "extremely rare" but potentially dangerous blood clots, all occurring in women between the ages of 18 and 48. Health officials said Friday that the number had risen to 15 women, three of whom died and seven were still hospitalized, out of nearly 8 million doses given.
In explaining the pause on that vaccine, Arwady said the clots "might" have something to do with estrogen.
"At this point, you know, six cases is really tiny compared to, again, the almost 7 million that have gotten this vaccine, but I don't think it's a surprise to see something that could be related to an immune reaction affecting women," Arwady said.
"We know that autoimmune diseases generally are much more prevalent in women. And that's because women and men do have differences in their immune system. Many of the immune modulating genes live on the X chromosome and if you remember back to high school biology, women have two X chromosomes and men only have one, they have X and Y, and so there's more opportunity for sort of different genetic things on X chromosomes in women as compared to men," she continued.
"But also, estrogen does tend to be a bit of an immune modulator in a positive way and then testosterone in a bit of a negative way, and because women need to, you know, evolutionarily be able to have babies, there are reasons that you don't, you know, you don't want their immune system rejecting a fetus, that kind of thing," Arwady said.
Like Clancy said her intent with collecting experiences with periods was in part to "help clinicians advise menstruators," Arwady said the pause on the Johnson & Johnson vaccine was likely recommended for a similar motivation: to inform doctors about the potential reaction and how to identify and treat it, noting that a medicine typically given to patients with blood clots could actually be dangerous in these cases if the person has low platelets, as the identified cases did.