Soon after the COVID-19 vaccines rolled out, some women reported brief changes to their menstrual cycles following a COVID-19 vaccine dose. Because there is often a lot of variability in period cycles, though, it wasn’t clear whether the shots necessarily caused the changes.
Formal studies investigating the phenomenon have since found that COVID-19 vaccination does appear to be associated with menstrual cycle alterations — but the changes are small and fleeting.
Multiple studies, for example, have now shown that a COVID-19 vaccine dose is associated with a short delay until a person’s next period — on average, less than one additional day — and that the delays are gone within the next cycle or two.
Experts who have conducted the studies have emphasized that the changes are not concerning, given the small effects and the fact that an abundance of other data show no effect of COVID-19 vaccination on female fertility. Still, women should be aware of the possibility, so they’re not surprised if their period is a bit different after they get vaccinated.
Despite the robust evidence, some people are pointing to the menstrual cycle changes to baselessly suggest that the COVID-19 vaccines are unsafe.
The latest online furor over the topic stems from an undercover video released on Feb. 2 by Project Veritas, a conservative activist group known for producing controversial undercover videos using deceptivepractices. In the video, a man named Jordon Trishton Walker, who is identified as a Pfizer director, alluded to menstrual cycle changes following vaccination.
“There is something irregular about their menstrual cycles,” he said, “So, people will have to investigate that down the line because that is a little concerning.” He went on to speculate about the cause of the cycle alterations, and said the vaccine “shouldn’t be interfering” with menstruation.
As we said, experts who have looked into this say it’s not concerning. But people opposed to COVID-19 vaccination quickly shared the video, which Project Veritas promoted as a Pfizer “Director Concerned Over Women’s Reproductive [Health] After COVID-19 Vaccinations.”
Rep. Marjorie Taylor Greene, a Republican from Georgia who has a history of spreading misinformation, retweeted the video, adding suggestively, “Many women have reported problems with their menstrual cycles after taking the vaccines. And women have reported miscarriages as well. We need answers and we will be asking.”
Miscarriage, which occurs in around 10% to 20% of pregnancies, is not more common after COVID-19 vaccination, as we’ve written. All of the available evidencesuggests the COVID-19 vaccines are safe during pregnancy and benefit both the mother and the fetus.
On the same day the Project Veritas video was released, other purveyors of COVID-19 misinformation also incorrectly concluded that if the vaccines alter menstrual cycles, then they must be harmful to female fertility.
“Menstrual changes even by one day in the cycle influence ovulation, fertilization, implantation, and uterine shedding,” wrote Dr. Peter McCullough, a cardiologist known for spreading COVID-19 vaccine misinformation, on Twitter. “So any perturbation will almost certainly lead to population decreases in fecundity and fertility.”
But Viki Male, a lecturer in reproductive immunology at Imperial College London who has studied menstrual changes after COVID-19 vaccination, said on Twitter that McCullough’s claim was “unfounded.”
Numerous studies have not found any effect of vaccination on female fertility. And one study, Male noted, found no reduction in fertility even for the menstrual cycle in which someone is vaccinated. “This makes sense since cycle delays – though they do happen – are in the order of a day, which is longer than the window of implantation in humans,” she added.
“Vaccines are meant to temporarily activate our immune system in order to recognize and help prevent or mitigate a future infection so we hypothesize that this temporary activation creates a temporary disturbance in the processes around menstruation which results in some individuals experiencing changes,” Dr. Alison Edelman, an obstetrician and gynecologist at Oregon Health & Science University who has also studied the menstrual change phenomenon, told us in an email.
“If someone experiences a change, research demonstrates that for the majority of people it does not appear to be long-lasting,” she added. “We also know that menstruation can be a sign of fertility health – we now have good evidence demonstrating no impact on the ability to get pregnant and that the vaccine is safe for pregnant individuals, which is a population at greater risk of severe illness and death from COVID19 infection.”
The video was one of several Project Veritas released with Walker, who purportedly thought he was on a date when he was secretly recorded. FactCheck.org was unable to verify whether Walker was ever a Pfizer employee, or whether he held the claimed title of “Director of Research and Development, Strategic Operations and mRNA Scientific Planning.” Pfizer has not responded to our inquiries about him.
A man with his name, however, graduated in 2018 from UT Southwestern Medical School, according to a graduation ceremony program available online. Tufts Medical Center told us he worked as a general surgery intern for a year, and Boston Consulting Group told us he was employed as a consultant from January 2020 until June 2021. This suggests Walker has broad medical training, but no specific knowledge of obstetrics and gynecology or vaccines.
Indeed, Male said on Twitter that given Walker’s uninformed comments in the video, “I doubt that this person (who the interviewer says is a urologist) has anything to do with vaccine development or safety.”
Menstrual Cycle Changes Following Vaccination
Studies have consistently found that COVID-19 vaccination is associated with a temporary, slightly longer time between periods.
One of the first studies, published last January and led by Edelman, analyzed period tracking app data from nearly 4,000 participants in the U.S. who had normal-length periods and were not using hormonal birth control. The volunteers were enrolled in the study prior to any COVID-19 vaccination, and about 60% of people were vaccinated
Because periods are naturally variable, the researchers looked at cycle length changes before and after vaccination for each person and compared them with unvaccinated people over the same amount of time. Vaccination was not associated with a change in bleeding time, but was associated with less than a one-day increase in cycle length, which rose to about two days for the small group of people who received two vaccine doses in the same menstrual cycle. The cycle length changes went away after one to two cycles.
A follow-up study, which Edelman and Male co-authored and published in September, used the same method for a global population of nearly 20,000 people, and found a less-than-one-day increase in cycle length that returned to normal within one cycle.
Another study, published in November, took a similar approach with period data inputted from almost 10,000 iPhone users, and identified a half-day or less increase in menstrual cycles after a COVID-19 dose. This increase disappeared within the cycle after vaccination.
“Menstrual cycle change following COVID-19 vaccination appears small and temporary and should not discourage individuals from becoming vaccinated,” the paper concluded.
A study of 79 people in the U.K., conducted by Male, also found vaccination associated with a small delay in the next period, which quickly returned to normal. The study, while small, did not identify any cycle length changes among vaccinated people using hormonal birth control.
Finally, a study of almost 4,000 nurses in North America, which surveyed people every six months, found vaccinated women had a 48% higher risk of reporting a longer cycle than unvaccinated women — a result that did not extend to the next survey.
A few studies have identified other possible temporary menstrual changes following vaccination, including increases in blood flow, but these are not as well studied.
Possible Mechanisms for Menstrual Changes
In one part of the Project Veritas video, Walker, whose remarks were edited, speculated about the mechanism for the menstrual cycle changes, saying “it has to be affecting something hormonal.” He added that “we need to figure out how it is impacting these hormones because the signaling starts in the brain” and “the vaccine doesn’t cross the blood-brain barrier.”
A hormone-based explanation is certainly one of the leading hypotheses for why vaccination can cause some small, temporary changes to menstrual cycles.
As Male laid out in a review published in Science in November, there are two plausible, non-mutually exclusive mechanisms: one involving the innate immune system’s interim ability to interfere with hormones, and another involving immune cells such as macrophages and natural killer cells, which help control the shedding and regrowth of the uterine lining.
Regardless, “nothing untoward is going on in the brain,” Male told us.
“We don’t yet know exactly how COVID vaccination is causing these small changes to menstrual timing and flow, but I think the most likely possibility is that aspects of the immune response can temporarily alter the way the hypothalamus, pituitary and ovaries communicate with each other (the HPO axis),” she said in an email. “To be clear, this doesn’t mean that the vaccine gets into or directly affects any of these organs (there is no evidence of this) but rather [that] they are responding to the general immune activation in your body.”
“We already know that this can happen in fever, which affects about 15% of people who are vaccinated,” Male continued. “As part of initiating the protective immune response, immune molecules called ‘cytokines’ are produced, and some of these reset the thermostat of the hypothalamus to increase your body temperature. It’s possible that a similar mechanism is at work to alter the timing of periods and one piece of evidence in support of this is that people whose cycles are driven by taking hormonal contraception seem to be less likely to notice a change than people whose cycles are being driven by their HPO axis.”
In her review, Male noted that menstrual changes have been reported before for other vaccines, and there is some evidence to indicate that such changes might occur after infection with the coronavirus. The studies on COVID-19, however, she said, have not been as rigorously done as for vaccination, in part because infection is much harder to study.
Edelman and Male agree more research is needed in the long-understudied area of menstrual health, not just for the COVID-19 vaccines, but also for other vaccines and diseases.
“[W]e need to do a better job of listening and validating the patient experience and integrate issues which are important to the public, like menstrual health, into future vaccine trials so we can better mitigate fears and concerns,” Edelman said.
But it’s misleading for others to appeal to such concerns to undermine vaccination, when a significant body of work shows menstrual cycle changes are minor and temporary.
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