Can COVID-19 Vaccines Affect Periods?

Several reports have been linked to COVID-19 vaccines that have showed changes to people’s menstrual cycles. What do we know about this potential link so far?

COVID-19 vaccines are arguably the world’s most important tool in the fight against the COVID-19 pandemic. Around the world, 19 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.

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.

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?

Heavy periods and breakthrough bleeding

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, and associate professor in the Department of Anthropology at the University of Illinois at Urbana-Champaign — two researchers who are currently investigating the link between COVID-19 vaccines and period changes.

After both having experienced some kind of change to their menstrual cycles after receiving their vaccines, Dr. Lee and Dr. Clancy both started investigating this phenomenon.

It 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. Katharine Lee

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.”

Dr. Clancy also noted, “for the most part, the most common outcome is actually nothing happening at all.”

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.

Dr. Kathryn Clancy

What might explain it, and who is at risk?

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.

Dr. Lee and Dr. 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

Dr. Clancy also adds 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.”

Having a high level of estrogen can also be one factor. Another factor is 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.

We’ve all been stressed since the start of this pandemic and well before. Any stress can affect our cortisol levels. Cortisol is known to affect ovulation and FSH/LH (follicle-stimulating hormone/luteinizing hormone) levels. So, 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? It’s possible.

The COVID-19 vaccines rely on your immune system to mount an immune response and produce antibodies that will protect you against the virus. If you have an exaggerated response or side effects, you could have undetected autoimmune issues.

Changes to menstrual periods also occur in unvaccinated people for different reasons. So, 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.

How long do these changes last?

Vaccine studies have shown that most of these changes are for the first few days following vaccination and quickly resolve. Consistent with this is when people report changes to their period, it is most commonly only in their immediate cycle, with subsequent cycles returning to baseline.

An advice from Dr. Kathleen Jordan, a specialist in internal medicine and infectious disease and the senior vice president of Medical Affairs at Tia Clinic.

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

Dr. Kathleen Jordan

All in all, the main thing is just to take care of yourself. Take it easy if you’re uncomfortable. If you are experiencing much more bleeding than expected, if you’re feeling faint, if you’re having an exceptionally heavy period, or if it’s lasting multiple weeks, you should go see a doctor.

There 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.

Women who experience any bleeding after menopause should also seek medical advice.

The benefits outweigh the potential risks.

Dr. Jordan, Dr. Lee, and Dr. Clancy, emphasized that the COVID-19 vaccines do not pose any risks to fertility. The Centers for Disease Control and Prevention (CDC) also clarified 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.

Dr. Kathryn Clancy

People who have sought medical advice for period changes, most of the 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.

Expectations for further research.

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 responses 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. Katharine Lee

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:

“It’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. But 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.

A majority of people wished they had more information about potential changes to her period before receiving the COVID-19 vaccine so that this effect would not take them by 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 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. 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.


Society should make periods a more general topic as it is still rarely talked about because many still see this as a taboo topic. This is an attitude that, and 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.


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.

As a woman, we are 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.

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