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Why were scientists so slow to study Covid-19 vaccines and menstruation?

How ignoring a key side effect of Covid-19 vaccines led to one of the most pervasive vaccination myths about fertility.

A vial of vaccine on a desk.
A Pfizer Covid-19 vaccine vial is visible on a desk before a Senate subcommittee hearing on transporting a coronavirus vaccine on December 10, 2020, in Washington, DC.
Andrew Harnik/Getty Images

The rollout of Covid-19 vaccines has laid bare some hard lessons, among them the need to fund the United States’ crumbling public health infrastructure and prioritize equitable access to high-quality health information. But it’s also drawn attention to some long-ignored problems in the way new vaccines and other medical products are studied — including the way researchers evaluate the effects of those products on menstrual cycles.

Like most clinical trials, the 2020 Covid-19 vaccine studies didn’t collect data about menstrual variability among participants. Younger people became eligible for vaccination in early 2021, and as they did, high-profile people — including former Pfizer executive Michael Yeadon and feminist author Naomi Wolf — raised concerns that vaccines could negatively affect reproductive health, often in the name of women’s wellness. Menstruating people worldwide began to ask questions about whether changes in their periods portended more serious changes in their current or future ability to conceive.

Soon after, myths linking Covid-19 vaccines with infertility began spreading with surprising ferocity — facilitated, say experts, by the absence of data addressing the menstrual side effects of vaccination.

Last month, the National Institutes of Health announced publication of a study it funded on menstrual changes associated with Covid-19 vaccination, the first peer-reviewed study of its kind. Earlier availability of this data might have enabled “prebunking” of vaccine-related fertility myths, dampening their spread and impact, but by the time the NIH noted in a press release that the results were reassuring, the damage had already been done.

In expert circles, the immune system is known to play a role in the menstrual cycle, and events that stress the immune system are known to cause menstrual changes. So while doctors and researchers weren’t particularly surprised to hear of minor, short-lived menstrual irregularities following vaccination, many seemed surprised by the public’s reaction to these side effects, and by the false narratives that sprang up to explain them.

Scholars of vaccine misinformation were less shocked. “A lot of misinformation has some kind of kernel of truth or is based on some kind of concern or question that is extremely valid,” said Shaydanay Urbani, who manages partnerships and programs at the misinformation research and education nonprofit First Draft. Countering that kind of information requires having good-quality data to inform people about what is true and what is not, she said.

Until recently, we did not have that data. Now, in addition to the study demonstrating a slight increase in the interval between periods, several preprint studies suggest many menstruating people likely have temporary changes in their flow after vaccination. These changes do not predict fertility problems: Multiple studies have shown that Covid-19 vaccination has no impact on the ability to conceive or safely carry a healthy pregnancy. Conversely, Covid-19 infection poses serious dangers to expectant parents and their pregnancies, especially for those who are unvaccinated.

Amid the many lessons we’ve learned about vaccine safety over the past two years, perhaps one of the most important is about designing future studies to avoid surges of fertility-related misinformation. “This can really be a call to action to include this type of data in future prospective clinical studies,” said Jason Wright, an OB-GYN at Columbia University and the editor-in-chief of the Obstetrics & Gynecology journal, which published the NIH study. “I hope this is the start of a bigger movement.”

Distrust in a data vacuum

It has only been 29 years since federal law required NIH-funded research to include women, and only rarely do studies of new vaccines or medications gather data related to menstrual changes. In 2015, the American College of Obstetricians and Gynecologists (ACOG) recommended that clinicians evaluate the menstrual cycle as the “fifth vital sign,” but assessments of menstrual outcomes as part of clinical trials are still vanishingly rare.

That’s in large part because of a concern gap between what matters to providers and what matters to patients. “Researchers just haven’t realized the importance of the menstrual cycle to women,” said Wright. While scientists may understand that the menstrual cycle is sensitive (and may vary in response) to sleep, smoking, nutrition, stress, and a host of other factors, changes in the intervals between periods and in the intensity of menstrual flow are often a mystery to individuals.

So while ACOG defines a normal interval between periods as ranging from 21 to 35 days, a two-day delay in period onset might feel wildly abnormal to someone trying to conceive or avoid pregnancy. And even if it occurred after taking a new medication or vaccine, that two-day delay might not register as a notable side effect to a researcher, making it unlikely to appear on the list of side effects and toxicities asked about in a study.

But failing to alert patients about a potential side effect like a change in a carefully tracked metric — for example, the timing of one’s menstrual period — can lead to fear. “If people across the board understood more, we’d be scared by a lot less,” said Brandi Shah, a Birmingham, Alabama-based family doctor who specializes in adolescent health.

Theoretically, if doctors gathered data on period changes — which clearly matter to patients — it could contribute to higher levels of public trust in medical science. But the right volume of data and the timing of its dissemination matter. “When you inundate people with good-quality information,” says Urbani, “that doesn’t necessarily mean that it’s going to change people’s behaviors or their decision-making.”

Sharing safety data about period changes early in the vaccines’ rollout might have inoculated people against later myths. While we’ve missed that opportunity, sharing this data now, especially in the visual forms and social networks where misinformation spreads fastest, can still have some corrective effect, says Urbani.

Emerging data suggests menstruating people are actually experiencing what they say they are experiencing

Three published studies have now examined vaccination-related menstrual changes in large groups of people receiving both mRNA vaccines (like Pfizer/BioNTech’s and Moderna’s) and vaccines made using virus-based technology (like Johnson & Johnson’s). This body of literature is small, and two of the published studies have not yet been peer-reviewed, meaning that they have not yet been scientifically vetted by a group of other researchers.

The studies were designed to evaluate different types of menstrual changes. While the NIH-funded study focused on changes to the length of the menstrual cycle, the other two studies also evaluated changes in flow on bleeding days and spotting between periods, and one assessed for bleeding in postmenopausal people. Of the three studies, only the peer-reviewed study included a control group of unvaccinated individuals.

The NIH-funded, peer-reviewed study looked at data people had been entering in an FDA-approved menstrual cycle tracking app since October 2020, a few months before the vaccine rollout began. (People agree that their de-identified data can be used for research when they register to use the app.)

People were prompted by the app to enter their vaccination date or confirm their unvaccinated status, and six cycles’ worth of data was assessed for each person — for vaccinated people, three cycles before and after getting their first vaccine. Ultimately, researchers analyzed data from nearly 4,000 menstruating people, about two-thirds of whom received a Covid-19 vaccine.

On average, the first period after receiving a first dose of vaccine occurred only seven-tenths of a day later than usual, indicating a minuscule but measurable lengthening of the interval between periods. No change was noted among unvaccinated people. The vast majority of participants (90 percent) received two-dose mRNA vaccines; periods were delayed by nine-tenths of a day after the second dose for those who got one.

The increase in the interval between periods was driven mostly by the people who received both vaccine doses within a single menstrual cycle, 11 percent of whose cycles increased by eight days or more. (In comparison, only 4 percent of unvaccinated people experienced this degree of increase in their menstrual cycles.)

It makes biological sense that the subset of people vaccinated twice in one cycle would have particularly variable intervals between periods, the investigators wrote. Two-dose vaccines are given three or four weeks apart; in order to have gotten both vaccines between periods, the first dose would’ve had to be received immediately after bleeding stopped. That phase of the cycle is particularly sensitive to illnesses and other stressors, and a catastrophic illness during this portion of the cycle (for example, severe Covid-19 infection) could permanently disrupt menstrual rhythms. However, for the smaller group experiencing eight or more days of variability, cycles returned to their usual lengths within two cycles after vaccination. The study did not identify an increase in bleeding days between or within any of the groups.

As period changes go, the cycle length is relatively easy to measure — everybody agrees on what constitutes a day. In contrast, measuring changes in flow is complicated by the variety of ways a heavier flow can surprise its owner: A heavier period can just be longer than a usual period, or it can require the use of more menstrual products for more days per period, or it can involve passing lots of clots with a period. Flow changes can also mean going from no flow — as is the experience of postmenopausal people or many of those who use hormonal IUDs for contraception or other reasons — to spotting.

Still, two studies have attempted to quantify and characterize changes in menstrual flow following vaccination. The first, with results published prior to peer review in October 2021, was designed as an emergent response to medical experts’ dismissive responses to reports of these changes, and to some patients’ conflation of short-term menstrual changes with long-term fertility harms. This study involved a survey of nearly 40,000 fully vaccinated people, including those regularly menstruating and those without regular periods (i.e., people on period-suppressing hormonal treatments and those who were post-menopausal).

More than 40 percent of the regularly menstruating respondents reported heavier bleeding after vaccination; a third of those also bled for longer. Furthermore, a surprisingly high proportion of people without regular periods had post-vaccination spotting or bleeding — 66 percent of postmenopausal people and 70 percent of those who were premenopausal — as did nearly 40 percent of people on gender-affirming hormones.

A second study, also published prior to peer review in mid-January, was prompted by a deluge of reports of post-vaccination menstrual irregularities to British and Norwegian vaccine safety assurance systems. In this study, investigators evaluated responses to a survey completed by nearly 5,700 Norwegian women ages 18 to 30 and already participating in a longer-term study observing pandemic effects on young adults. In October 2021, after having received two vaccine doses, participants were asked to recall whether they’d had a variety of menstrual symptoms during their cycles immediately preceding and following their first and second vaccine doses. Participants reported nearly twice as much heavy bleeding after their first dose of vaccination as before, and, in general, more menstrual disturbances after their second dose. There was no assessment of a return to normal menstruation.

Together, these and a handful of other, differently focused studies prove that the symptoms so many have noticed are real and relevant to people who have periods — and affirm there’s both room and reason to improve how we assess menstrual side effects of new pharmaceuticals.

The way forward

More data on post-vaccination menstrual changes is likely to emerge soon: Alison Edelman, an OB-GYN at Oregon Health and Science University who led the NIH study, said her team had gathered and was still analyzing data on flow changes following vaccination, and the European Medicines Agency recently requested an “in-depth evaluation” of available data on post-vaccination menstrual changes.

Already, a paradigm shift may be underway: Researchers designing early trials of a novel, non-Covid vaccine recently contacted Edelman about how to track menstrual changes among participants in their study. That seems like a step in the right direction. Unpleasant events are most worrisome when they come without warning, said Edelman — it’s why airlines warn passengers about airplane turbulence, and health providers warn Covid vaccine recipients about post-vaccination fevers.

Why not menstrual changes, too? “I think it’s important that we have that information there, especially for something that people experience so frequently,” she says.

Jason Wright, the editor of the journal that published Edelman’s study, also hopes her team’s findings spur broader efforts to build assessments of menstrual side effects into future research. For now, he says, they enable doctors to provide counseling about changes in advance of vaccinations, and to reassure patients that the subject has been studied and is important to providers. “The more solid information that we can talk to patients about, I think, the better,” he says.