There have been several reports linking COVID-19 vaccines to changes to people’s menstrual cycles. What do we know about this potential link so far? Medical News Today has spoken with researchers, physicians, and people who have experienced changes to their own cycles after receiving their vaccines to find out.

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Many people have reported changes to their periods after receiving a COVID-19 vaccine. What do experts have to say about this phenomenon? CHANDAN KHANNA/AFP via Getty Images

All data and statistics are based on publicly available data at the time of publication. Some information may be out of date.

COVID-19 vaccines are arguably the world’s most important tool in the fight against the COVID-19 pandemic. Around the world, 20 vaccines have received emergency use authorization from the relevant regulatory authorities in at least one country.

However, one issue has continued to agitate the minds of the general public and health experts alike: What side effects might these vaccines cause, how often, and under what circumstances?

Commonly reported side effects across the different types of vaccines include fevers, fatigue, headaches, and body aches.

Stay informed with live updates on the current COVID-19 outbreak and visit our coronavirus hub for more advice on prevention and treatment.

Serious side effects are extremely rare, and national and international health agencies continue to collect and monitor reports about any adverse reactions.

However, as vaccination rollouts have progressed around the world, some people have pointed out a potential side effect that feeds into existing debates about the gender data gap in medical research: changes to the menstrual cycle.

There have been many anecdotal reports of changes to people’s menstrual cycles after receiving a COVID-19 vaccine, yet specific data about this phenomenon’s frequency are currently scarce.

Information that The Times obtained indicates that in the United Kingdom, the Medicines & Healthcare Products Regulatory Agency received almost 4,000 reports of changes to people’s periods after a COVID-19 vaccine by May 17, 2021.

Of these, 2,734 cases occurred after the Oxford-AstraZeneca vaccine, 1,158 occurred after the Pfizer-BioNTech vaccine, and 66 occurred after the Moderna vaccine.

Due to these reports, many questions have arisen. How might a person’s menstrual cycle change after a vaccine? Are these really COVID-19-related side effects, or are they due to stress and other life changes that might coincide with getting the vaccine?

To find out more, MNT has spoken with four women with lived experiences of changes to their periods after getting a COVID-19 vaccine.*

We have also spoken with the two researchers who are currently investigating the link between COVID-19 vaccines and period changes: Dr. Katharine Lee, a postdoctoral research fellow in the Division of Public Health Sciences at the Washington University School of Medicine in St. Louis, MO, and Dr. Kathryn Clancy, an associate professor in the Department of Anthropology at the University of Illinois at Urbana-Champaign.

We also sought the informed opinions of two medical professionals: Dr. Tara Scott, an obstetrician-gynecologist and the founder of Revitalize, which is a functional medicine group focusing on women’s health, and Dr. Kathleen Jordan, a specialist in internal medicine and infectious disease and the senior vice president of Medical Affairs at Tia Clinic.

Article summary:

Drs. Lee and Clancy decided to start investigating the phenomenon of period changes after a COVID-19 vaccine after they both experienced some kind of change to their menstrual cycles after receiving their own vaccines.

“[I]t happened to me first, and I reached out to some of my friends who I knew were vaccinated and asked them if they’d noticed anything [after their COVID-19 vaccine], and a few people noted that their period was a little bit worse than usual […], or people who normally don’t have a period [were] noting that they had cramps or a little bit of spotting, which they would normally not have […],” Dr. Lee told us.

When Dr. Clancy also experienced period changes after her vaccine, she shared her experience in a Twitter thread, which quickly gained traction. Afterward, Drs. Lee and Clancy set up an online survey to collect as much self-reported data as possible about the menstrual cycle-related reactions that people were experiencing after COVID-19 vaccines. Their research is ongoing.

The researchers do not have data on how frequently period changes might occur among those who receive a COVID-19 vaccine, and they also caution that experiencing such changes “is not universal, just as getting fever and headache [isn’t a] universal [reaction to] the vaccine.”

In fact, Dr. Clancy noted, judging by the preliminary data that they were able to gather, “for the most part, the most common [outcome] […] is actually nothing happening at all.”

However, “among the people who are experiencing this side effect, it seems like the most common is — for people who are currently menstruating […] — [that] their period is heavier, sometimes longer, [and] for people who are not currently menstruating because they’re on long-acting contraceptives or they’re transgender and [on] gender-affirming hormones, or they are postmenopausal […], we’re also seeing breakthrough bleeding as another phenomenon.”

MNT also heard from regularly menstruating people who experienced heavier or unusual periods after getting their vaccines.

Sabrina, who is in her 40s, experienced spotting for 2 weeks after receiving her first COVID-19 vaccine. She then got a very heavy period.

“My periods are usually on the dot every 30 days and quite light,” she told MNT. “The month following my first jab, I had spotting for 2 weeks then the heaviest period I have had since my 20s, literally flooding through tampons [and sanitary] towels.”

Since then, she has been experiencing light bleeding between periods and heavier bleeding at the time when she would normally get her period.

Another reader, Louise, wrote to MNT to say that she had experienced “the worst period of [her] life” after receiving an Oxford-AstraZeneca vaccine.

“I could only liken it to post-childbirth. Seven full days on heavy absorbency [period products]. On days 2–3, I was bleeding through after about an hour, with really heavy clots. Usually, I would only need to change [my period product] about every 4 hours, and [my period] would only last 4–5 days.”

– Louise

Adrienne, who is also in her 40s and has received the Oxford-AstraZeneca vaccine, said she experienced a heavier period and more intense cramps after her first vaccine. After her second vaccine, she experienced premenstrual symptoms earlier than usual, then a heavier period again.

Lindy, who is 24 years old and has also received the Oxford-AstraZeneca vaccine, experienced unexpected changes to her menstrual cycle after receiving her second vaccine.

“I have an IUD [intrauterine device] (Mirena), so while my cycles are very regular, my periods are incredibly light,” she told MNT. “Normally, I get a tiny bit of spotting, and that’s it. About 2 weeks after I got my second jab, I had moderately heavy bleeding, which really caught me off guard. It stopped after a couple of days.”

“My next period was still heavier than normal, and it was also a few weeks late, which I thought was strange — I’m normally really regular, and I track my cycles with an app. I didn’t have any noticeable changes in [premenstrual] symptoms or cramping,” Lindy reported.

So far, it remains unclear what the biological mechanisms behind these period changes might be and who might be more at risk of experiencing them.

Drs. Lee and Clancy are yet to figure out whether or not there are any factors associated with the likelihood of going through a menstrual cycle change after getting a COVID-19 vaccine. However, Dr. Clancy noted that they are considering some hypotheses.

“If I were to make a guess, I would say [that] if somebody already has a disorder that might affect bleeding and clotting or has had issues with bleeding and clotting in the past, […] that’s a reason to at least talk to your doctor first if you haven’t gotten the vaccine yet, just to see if they have thoughts about whether one vaccine [is] better than another [in terms of mitigating any risks of side effects].”

– Dr. Kathryn Clancy

She also noted that “there is a small chance that bodies that have more endometrial practice — like bodies that have had a lot more menstrual cycles, basically, so older people, people […] who have been pregnant, birthing people themselves — […] there’s a chance that those bodies might be slightly more likely to have heavier periods [after a vaccine], simply because the vasculature of the uterus is going to be a lot more established in [them].”

Dr. Scott also hypothesized that a person’s unique hormone “cocktail” may play a role in how they experience periods after getting a vaccine.

Having a high level of estrogen, she said, might be one factor. “This is more common in women over 40 and is a result of the increased signal from the brain needed to [stimulate ovulation].”

She also suggested that cortisol, the so-called stress hormone, could affect periods and that the changes to menstrual cycles may not be in response to the COVID-19 vaccines but to increased stress levels.

“Many of us have been stressed since the start of this pandemic and well before,” Dr. Scott emphasized.

Dr. Jordan also highlighted the role that stress can play in affecting periods. “[A]ny stress can affect our cortisol levels,” she told MNT, explaining that “cortisol is known to affect ovulation and FSH/LH [follicle-stimulating hormone/luteinizing hormone] levels.”

“Could the stress of the vaccine or other stressors of the pandemic itself trigger changes in our cortisol levels, which then, in turn, affect our other hormones and menses? Possibly,” she suggested.

Dr. Scott further theorized that underlying autoimmune issues might be to blame in some cases: “The [COVID-19 vaccines rely] on your immune system to mount an immune response and produce antibodies that will protect you against the [SARS-CoV-2] virus. If you had an exaggerated response or side effects, you could have undetected autoimmune issues.”

Dr. Jordan, however, said that changes to menstrual periods also occur in unvaccinated people for different reasons. “[S]o, we have no way of knowing if there is anything specifically caused by the vaccination or if this is occurring at background rates” — that is, if the so-called reactions may, in many cases, be coincidental.

According to Dr. Jordan, “vaccine studies have shown that most of these changes are for the first few days following vaccination and quickly resolve.”

“Consistent with this is that when [people] report changes to their period, it is most commonly only in their immediate cycle, with subsequent cycles returning to baseline,” she added.

Dr. Jordan’s advice to readers who do experience changes to their menstrual cycles after receiving a vaccine was as follows:

“It depends on what the change is … for any missed periods, always check a pregnancy test — after all, common things are still common! If you are experiencing pain [or] significant or persistent changes in your menses, check in with a healthcare professional. Our cycles are biologically complex, so a variety of things can affect them, and your healthcare provider can evaluate. I would also reassure anyone who has just experienced an abnormal period immediately following vaccination that there is now fairly large-scale evidence that there is no ill effect on fertility or pregnancy — and patterns suggest their subsequent cycles should normalize.”

“[T]he main thing is just [to] take care of yourself. [T]ake it easy if you’re uncomfortable,” Dr. Clancy also advised. “[I]f you are experiencing much more [bleeding] than expected, [if] […] you’re feeling faint, […] [or] if you’re having an exceptionally heavy period or it’s lasting multiple weeks, you should go see a doctor,” she added.

“[T]here are multiple methods by which doctors can help stop bleeding, […] but there are also some things that they can give you that will actually help you clot a little bit better,” Dr. Clancy said.

She also advised women who experience any bleeding after menopause to seek medical advice.

Dr. Jordan, as well as Drs. Lee and Clancy, emphasized that the COVID-19 vaccines do not pose any risks to fertility. The Centers for Disease Control and Prevention (CDC) also clarify that there is no evidence to suggest that these vaccines could affect a person’s fertility.

“I want to be clear that we have no reason to think that this is going to affect fertility,” Dr. Lee emphasized. “In fact, there are people who have gotten pregnant who thought their period was missing, but actually it wasn’t, they were pregnant, and they reached out to us again to say ‘it’s not that it was late, I’m pregnant,’” she added.

“And [I] also [want to add] that getting [COVID-19] is far, far worse for your long-term health, for future fertility, for your period.”

– Dr. Katharine Lee

Dr. Clancy also said that regardless of the potential effects on the menstrual cycle, she would not hesitate to get the COVID-19 vaccine again, thanks to the protection it offers to the individual and the community.

“I would get it again, and I would happily have worse symptoms than what I had in order to be protected” and to protect her loved ones, she told MNT.

Dr. Jordan also told us that among those who have sought medical advice for period changes at her clinic, “by and large, [their] more common comments about the vaccine are of appreciation for newfound ‘freedoms’ to socialize again — the vaccine being their ticket to seeing friends and family again, traveling, and returning to in-person work or school.”

However, in speaking with MNT, both the people with lived experiences of period changes after getting a COVID-19 vaccine and the researchers investigating this phenomenon flagged the stringent need to include menstruating people in clinical trials, record any period-related effects, and keep the public informed of any such phenomena.

“I think just remembering to ask about differences in the menstrual cycle as part of standard clinical testing of vaccines might be nice, given that we expect a huge immune response, and we know that huge immune response[s] can disrupt lots of other inflammatory pathways in people, and menstrual cycles tend to be something that people who have periods pay attention to, and [they] notice when things get a little bit wonky […],” Dr. Lee told us.

Dr. Lee also expressed some disappointment that the researchers assessing the safety and effectiveness of two-dose COVID-19 vaccines appeared not to have considered assessments of their potential impact on menstrual cycles:

“[I]t’s two doses spread approximately a menstrual cycle apart for a lot of folks, and so to not even think to ask [about periods], in hindsight, seems like an oversight. [B]ut also there’s the whole history of vaccine trials and clinical trials to contend with, where women and people who are not white men, basically, were left out of clinical trials for a really, really long time. And even to this day, a lot of times, [researchers] will exclude […] people who may become pregnant out of fear that something could happen to a fetus that does not yet exist.”

This is a premise that researchers and health experts ought to rethink going forward, Dr. Lee suggested.

Adrienne also told MNT that she wished she had more information about potential changes to her period before receiving the COVID-19 vaccine so that this effect did not take her by as much surprise.

“I guess it would have been good to be prepared for it ahead of time and for the scientific community to take this impact seriously, as women do tend to just suffer through it,” she said.

“I feel like if the vaccine were making men’s testicles sore, we would all know about it, and they would probably be looking into it pretty quickly! I also wonder why it is impacting on periods and feel we do need to know more about it. […] Women’s health needs specific attention and research.”

– Adrienne

Also, Louise spoke about how periods are rarely talked about because many still see them as a taboo topic. This is an attitude that, as a society, we all need to change in order to help improve the quality of life of all people who have menstrual cycles.

“I think so many of us have been brought up to think about periods as a kind of personal, private, unpleasant thing we just have to put up with, and it doesn’t always feel intuitive to talk to friends about weird changes you’ve experienced or how much pain you’re in,” said Louise.

“There’s a sort of weird ‘pride’ we have in being strong enough to deal with so much pain — I know it’s not like this for everyone, but I’ve definitely seen plenty of eye rolls at girls who had to leave school when they had awful [premenstrual symptoms] or had to call in sick to work because their periods were so bad.”

“We’re just expected to shut up and put up with it when, in fact, it’s important to talk about it because it allows us to see patterns in what we’re all experiencing,” she observed.

* We have changed the names of these contributors to protect their identities.

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