First study of its kind reports on period changes post-Covid-19 vaccination

Nikita used to joke she was like the moon because of her clockwork-like menstrual cycle. “My cycle was so regular,” the 32-year-old Londoner tells Inverse. After her first dose of the Pfizer vaccine in July 2021, that regularity changed.

Her period came a few days late, heavier than usual. Even her most recent period, she says, was 18 days late. “I was just freaking out, I took so many pregnancy tests,” says Nikita, who works in human rights. This hasn’t stopped her from finishing the original two-dose series or getting Pfizer’s booster when it was recommended.

Nylah Burton, 27, also had “extremely regular” cycles until her second dose of the Pfizer vaccine. Burton, a journalist, says her period after that jab came three days late with cramping, which is atypical for her. Her following period arrived three days early, sans cramps, and then her regularity resumed. After her third vaccine, Burton had the same two-cycle hiccup.

Nikita and Burton are a few of many people who observed that right after receiving the Covid-19 vaccine — regardless of the brand — they had at least one abnormal period. For some, their periods reverted after a few cycles, but others have had to adjust to a completely new one.

Biological anthropologist and postdoctoral fellow at Washington University Katharine Lee also got a “wonky period” after Covid vaccination. So she called fellow biological anthropologist Kathryn Clancy about it, who had the same experience. They set to work investigating.

Their work was published today in the journal Science Advances. They surveyed more than 39,000 currently or formerly menstruating people. 42 percent reported wonky periods.

What’s new — This is the first systematic survey published to examine menstrual anomalies and the Covid-19 vaccine. Lee and her team issued the survey between April and October of 2021 when folks scheduled their first and second vaccine in droves. They recorded responses from 39,129 people.

The survey shows some were more likely to have post-vaccine irregular periods than others. Those who had given birth or been pregnant previously were more prone to heavier bleeding, as were Hispanic or Latin people. Those with conditions like endometriosis and polycystic ovary syndrome (PCOS), both of which affect the menstrual cycle, were also more likely to bleed heavier than usual.

“Not knowing this was going to happen...not being prepared for it to happen meant it was an unnecessarily scary experience for many people”

Some respondents who hadn’t had a period in months or years saw blood. Those with long-acting reversible contraceptives, like hormonal intrauterine devices (IUDs) or implants, had their first period in a long while, as did those on testosterone for gender-affirming therapy. Even those who had reached menopause — for whom spotting can be a sign of cancer — had what’s known as breakthrough bleeding.

“Not knowing this was going to happen, not knowing the mechanism by which it happens, and not being prepared for it to happen meant it was an unnecessarily scary experience for many people,” Lee tells Inverse.

“It ended up making a lot of patients very mad,” she continues. “It weakens trust in medicine. It weakens your relationship with patients, and it feeds into this mistrusting of medicine, of the government, of pharma companies, all of which are really necessary for a functioning society.”

Why it matters — This survey reveals how public health regards menstruation. Its significance isn’t in showing a cause and effect — it takes great care to avoid that implication — because it simply shows a number of people had a similar experience without proving a causal link.

First, it questions clinical trial protocol. The paper notes that follow-up for vaccines usually lasts seven days without an additional check on menstrual cycles. Granted, Covid-19 vaccines received Emergency Use Authorization, so researchers likely weren’t going to tack on more time if they didn’t have to. But perhaps it’s time for the protocol to change.

“I think that now this warrants a formal study as part of vaccine trials with monitoring of reproductive outcomes,” says Andrea Dunaif, an endocrinologist at Mount Sinai. Dunaif was not involved in the survey.

The vaccines are in phase 4 of clinical testing, what’s known as post-marketing. Now that it’s circulating among billions of people, researchers can observe its effects on a massive scale among various populations. This phase monitors large-scale potential side-effects such as this one, but Lee says it may have been useful to know about it beforehand. Just as patients stocked up on Tylenol and easy food to prepare for the vaccine’s fever and aches, they could have adequately prepared for a wonky period.

The Centers for Disease Control (CDC) pay attention to these flares, known as “adverse events of special interest (AESIs).” The CDC’s Immunization Safety Office told Inverse, “In the case of menstrual irregularities, CDC is aware of reports of such irregularities, and has conducted analyses of such reports. Those data are planned for publication in a scientific journal soon.” What’s more, the CDC only gets involved with vaccines once they’re ready for EUA.

“Medicine doesn't pay attention to women's menstrual cycles, and the whole push that menses are a vital sign is incredibly important”

Second, it signals that health experts haven’t viewed menstruation as consequential to health. Since monitoring impacts on menstruation isn’t part of early vaccine testing, research lacks any possibly useful data on periods. This bias can show in the examination room too.

“Medicine doesn't pay attention to women's menstrual cycles, and the whole push that menses are a vital sign is incredibly important,” says Dunaif.

Lee notes that some went to their doctor about abnormal periods only to be dismissed. While this cavalier attitude speaks to medical misogyny, it’s also an unfortunate consequence of not equipping doctors with all the information.

“Not being listed as a side effect is one reason why people went to their doctors and their doctors said, ‘That's not a side effect,’ because they look at the list, right?” says Lee. “So if the list isn’t inclusive, then that feeds into people not being believed.”

Third, it demonstrates that periods aren’t equal. What’s an inconvenience to some may be debilitating for others. Christine, a 45-year-old in Orange County, California, already endured periods so heavy that she had to get iron infusions at the hospital. She got her first shot in August 2021, and she says that from then until December her cycle was even worse.

The survey notes that for menstruating trans and non-binary people taking testosterone, an unexpected period brings risks of gender dysphoria or having to address the problem in public.

However, Nina, a 25-year-old lawyer in New Jersey, had an unusual albeit positive experience. For a few years, she got her period about every six months, which she attributes to stress and an eating disorder. Just when she scheduled an ultrasound to see if anything was seriously wrong with her uterus, she got her first vaccine in January 2021 — followed by her first period in months. It’s been regular ever since. “I was just really excited,” she tells Inverse.

Digging into the details — The menstrual and reproductive system is quite good for indicating whether something’s off. Lee says one’s period is meant to respond to stimuli like stress to the immune system.

On the one hand, that means nearly anything from medication to stress can affect the menstrual cycle. While it’s notable that menstruation changed for so many people after a Covid-19 vaccine, it’s crucial to look at all the details and to remember that everyone’s struggling with the pandemic.

Dunaif acknowledges the myriad factors that influence menstruation. Still, she says the researchers were rigorous enough that their findings aren’t unfounded.

On the other hand, a period’s responsiveness suggests that menstrual tracking is crucial to vaccine development. Dunaif also points out that with a plethora of period-tracking apps, it would be easy to include accurate, self-reported data.

Furthermore, there’s some fear that discussing these experiences will deter vaccination. “I've definitely heard some backlash for mentioning changes to your period that implies you’re anti- vaccine and I'm like, ‘No, I'm definitely, definitely pro-vaccine,’” says Elizabeth Esparza, a 29-year-old in Oakland, California who works for a nonprofit.

Still, these unlisted potential side-effects impact people’s decisions to get vaccinated. Nikita in London, who in addition to irregular cycles experienced hours of vomiting from her third jab, has decided she’s done with Covid vaccines. Christine in California says the vaccine’s effect on her menstrual cycle “took things in such a direction that I didn't want to put a second shot in my body.”

Some question whether this side-effect means the shot impacts fertility. Lee and Dunaif stress that there’s ample evidence showing the vaccine doesn’t harm fertility. “There’s a difference between safety and side effects,” Lee says. “However, [fertility] is affected by Covid.”

There’s also no evidence that shifts in a menstrual cycle are inherently dangerous, but there needs to be a better understanding of why these shifts happen.

Lee’s study cites other studies documenting inoculations and unusual periods, such as a 1913 paper on the typhoid vaccine and studies on human papillomavirus and hepatitis B vaccines.

She and her team are curious about what biological mechanisms may create these aberrant periods and suspect it comes from immune-activated inflammation. The vaccine impels a huge immune response, which involves inflammatory processes like bleeding and clotting. This response could disrupt the body’s natural cycle of growing and shedding tissue every month.

What’s next — Plenty of people with wonky periods intend to get all the Covid-19 vaccines they need. Lee and her team hope for more funding to continue conducting surveys or look into the biological mechanisms behind the vaccine’s immune response and menstruation.

In the meantime, she hopes that clinicians take these concerns more seriously. “Starting from a place of listening and believing is always really important when people are sharing sensitive information,” she says.