Medical Examiner

Why We Don’t Know What’s Actually Going On With Periods and COVID Vaccines

Dr. Jen Gunter discusses a possible side effect and how it could have been overlooked.

A woman in a mask gets a COVID-`19 vaccine.
Hector Pertuz/iStock/Getty Images Plus

A conversation with some period-having friends spawned this story. We’d all received our vaccinations against COVID-19 and though everyone was at a different spot in their cycle, we’d all noticed that something was different. Our flows were either heavier than usual, or our periods were late, or we were having breakthrough bleeding, or the cramps were more painful, or some combination of it all. The only thing that had changed in anyone’s life was receiving a vaccine.

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Period irregularities can be triggered by a variety of normal events—including diet changes, stress, overexercising, taking medications like birth control and steroids, or even bad sleep. A variety of serious, underlying medical conditions could play a role, too, such as endometriosis, pelvic inflammatory disease, polycystic ovary syndrome, or uterine fibroids.

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Right now, there’s no scientific evidence that suggests the COVID-19 vaccines are causing irregular menstruation. Kate Clancy from the University of Illinois launched a survey collecting data on this earlier this month and shared a Twitter thread explaining how her period was heavier than usual post-vaccination. But there’s still a stunning lack of research into how periods interact with anything other than fertility or contraception, despite the role the endometrium plays in the immune system.

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Jen Gunter, an OB-GYN in the San Francisco Bay area, explored the possibilities for why this is happening in her newsletter, the Vajenda. After reading it and sending it to all of my friends, I spoke to Gunter about this phenomenon, why the menstrual cycle is an important indicator of health, and how the lack of data available inadvertently fuels conspiracy theories.

This interview has been edited and condensed for clarity.

Julia Craven: What have you been hearing about how vaccines affect the menstrual cycle? Because me and some of my friends who have periods were talking about how our cycles were just a tad bit different after we got vaccinated.

Jen Gunter: I’ve heard people who’ve had late periods, early periods, heavier periods than they expected. It’s more so earlier, more unexpected bleeding as opposed to a delay. That seems to be the common theme, or a more painful period. None of it has been what we would consider “flooding,” which would require going to the emergency department.

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I was reading your Substack and I never knew that the endometrium was a part of the immune system. Could you give us a condensed version of the role that our endometria play in immune response?

There’s a lot of complexity and stuff that happens with menstruation. Your immune system is involved in wound healing, and think about what happens every month with shedding your lining, and then rebuilding. Your endometrium is also connected with the outside world. Your vagina’s got bacteria—and your endometrium actually has a microbiome, although it’s not really well understood. So it has to be able to defend itself. We’ve evolved to deal with exposure to infections, sexually transmitted diseases, or whatnot. And the immune system’s got a super important role in implantation and pregnancy. So it’s a very active part of the immune system.

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And despite that, there was really very little information on any sort of impact in vaccination.

Was the potential effect on the menstrual cycle studied during the vaccine trials?

Not that I can tell. It wasn’t released in the data that was published. Now, does that mean they didn’t track it? I don’t know. It’s possible they have the data. But I haven’t been able to find it anywhere in the published studies.

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I did deep-dive into previous vaccinations because I thought, well, maybe somebody else tracked it. Obviously you can’t compare one vaccine to another because vaccines stimulate immune systems in different ways. There are two studies that talk about abnormal menstruation after the HPV vaccine, but those are post-marketing surveillance types of things. They’re not collected in real time.

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I think that we can say that if somebody were having menstruation that took them to the emergency department, that would absolutely be flagged in a vaccine study. So it’s clear that that hasn’t happened or that would be coming out in the reports, but that doesn’t help us with a period coming early or a period being heavy a few days after the vaccine. It’s certainly plausible that could happen, just like your lymph nodes might get swollen. It’s plausible that some vaccines might be more triggering for the lining of the uterus than others. And if we had some prospective data, we’d be able to answer those questions.

One thing that I’ve noticed from covering health is that the menstrual cycle doesn’t come up in a lot of studies or in a lot of discourse. And I wonder if you had thoughts on what that says about how medicine more broadly sees, or doesn’t see, the menstrual cycle, and how it’s potentially affected, as a valid indicator of health.

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In OB-GYN, it’s very frequently studied because we’re often giving people hormonal medications so menstrual cycle abnormalities are generally collected when we’re talking about a study that’s geared toward the uterus and the ovaries. But I think the further that we get away from that, that seems to be less collected. There’s probably a lot of different reasons for it. Many times, unfortunately, studies try to get a very homogeneous population. And if you have people who are at different points in their menstrual cycle every day of the month—cause that’s what happens, it changes every day—you don’t have a homogeneous population. Many people are on contraception who go into new drug studies and sometimes that’s a requirement, because it’s a new drug and we don’t know how it could impact pregnancy.

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And a lot of people probably think, “Well, as long as we’re picking up emergency room visits, then that’s enough.” From a drug safety standpoint, it probably is enough. But when dealing with people in real-life office situations, it’s not enough. People want to know what’s happening to their bodies. It’s really understandable because it’s frightening.

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For example, when we do nerve blocks, we give a large amount of steroids and the injection can trigger abnormal menstruation for people, and knowing about that means you can warn them. So then when it happens, they’re not freaking out. And they accept that as part of their informed consent. It’s a three-minute addition to the discussion.

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In the same way, when we tell people they might have a low-grade fever after a vaccine, if it happens they’re like, “Eh, no biggie. I knew I was going to get a low-grade fever, or it could happen.”

We need to expand the list of what we consider to be minor—and when I say minor I mean medically minor, I’m not trying to downplay people’s symptoms—complications related to medications, including vaccines, so that it’s collected. We’ve seen what happens without that information. Now we have anti-vaxxers running wild with it, saying that it’s causing miscarriages. So there’s not only the downstream effect of people wanting to know what happens to their body, but it’s becoming part of conspiracy theories and we’re seeing big people spreading it.

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Why would it have been good to know this was a possibility going into getting vaccinated?

People deserve to know what’s happening to their body so they’re not scared. If we think that a fever of 100, which is a low-grade fever, is worth recording, then an early period is worth recording. We shouldn’t be just looking for safety signals that are extreme. And I don’t think your period a few days early is a concern at all as far as long-term repercussions go, but people deserve to know what’s happening to their body.

We’re in a time of great stress, so adding one more unknown on top of that—people can only take so much. We have with medicine this history of saying, “There, there dear. You don’t need to know about that.”

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Then I think people start thinking about not only do they not know what’s happening to their body, but: “Well then what else don’t I know about the vaccine? What else are you hiding from me?” And then people can start thinking about things like DES or thalidomide or the Rely tampon. And then they could think the vaccine is more like that when it’s not at all. The safety issues with those were related to people not doing the right studies and due diligence and not responding when they should have responded. With that history of women being harmed, you can see how people go there.

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What are your thoughts on the conspiracies that are floating around now about infertility?

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We’ve seen this before with the HPV vaccine. The theories are based on absolute perversion of a few scientific phrases. This tactic works, because the conspiracy that the HPV vaccine causes infertility was a big thing when it came out and it took a lot to undo. And I think sometimes researchers don’t appreciate the depth of the conspiracy theories. So they don’t design their studies to protect against them. You’re designing a study to get a scientific answer. You’re not designing a study to shoot down a theory that should never have been advanced because it’s not valid.

Vaccine researchers need to actually spend some time talking with people who spend a lot of time dealing with conspiracy theories so if there’s a way to design a trial to be able to preempt some of that—if it’s possible, then it’d be nice.

I’ve spent the pandemic covering the coronavirus as a reporter and an editor. Slate Plus helps support everything from explainers on how to keep yourself safe (without unduly panicking) to our Diaries series, about how the virus is affecting our lives. We couldn’t do it without you. —Shannon Palus, senior editor

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