COVID-19 vaccines could temporarily change menstrual cycle, new study finds

After a COVID-19 vaccination, some of the most commonly reported side effects are fever, headache, fatigue and pain at the injection site. Vaccine experts say these are normal, short-lived and a sign that a person’s immune system is working and building protection against COVID-19.

But since the COVID vaccines became available, some women of reproductive age also began noticing changes in their menstrual cycles — particularly in length — shortly after receiving the jab. This led anti-vaxxers to speculate that this was a sign the vaccines could affect a woman’s fertility long term. Now new research is confirming that slight changes in menstrual cycle length can indeed be another potential side effect of a COVID-19 vaccine, but like other side effects, these changes appear to be only temporary.

A study published in BMJ Medicine last week found that it is possible for COVID vaccinations to slightly alter the menstrual cycle and cause an average increase in its length of less than one day. A menstrual cycle is measured from the first day of a person’s period to the first day of their next period.

“Menstruation is woefully understudied, which is troubling considering it is a key indicator of fertility and overall health,” Dr. Alison Edelman, a professor of obstetrics and gynecology at Oregon Health & Science University School of Medicine and the paper’s lead author, told Yahoo News in an email. “These findings are important because they further validate what so many individuals reported experiencing ... and allow health care professionals to provide patients with better care and clinical recommendations.”

For the study, Edelman and her colleagues analyzed data from nearly 20,000 people from 18 to 45 years old and with what is considered a normal cycle length, which is anywhere between 24 and 38 days, according to the research. About 15,000 of those studied were vaccinated, and approximately 5,000 were not. All used the period tracking app called Natural Cycles.

After analyzing the participants’ period data from three consecutive menstrual cycles prior to vaccination and at least one cycle post-vaccination, the researchers found that periods among those who received one dose of vaccine in one menstrual cycle were delayed on average by nearly one day. For those who received two shots within the same menstrual cycle, a bigger delay was observed. For this group, there was a larger increase in cycle length of almost four days.

For both groups, however, periods returned to pre-vaccination lengths in the cycle following vaccination. Additionally, the delays observed in vaccinated people’s periods were within the normal range, which Edelman said is a reassuring sign that the COVID-19 vaccines are unlikely to affect a woman’s fertility.

“If someone experiences a change, emerging research demonstrates that it does not appear long-lasting,” she said, adding that “we now have good evidence demonstrating no impact on the ability to get pregnant and that the vaccine is safe for pregnant individuals.”

The research, which was funded by the National Institutes of Health, included participants from the U.S. and Canada, as well as the United Kingdom and Europe, who had received different types of COVID vaccines. This allowed researchers to study the effects of not only the mRNA vaccines offered by Pfizer and Moderna, which are most widely used in the U.S., but also other COVID-19 shots that use different vaccine technologies. These include the AstraZeneca, Sinovac and Covaxin vaccines that are currently being used by other countries. According to the study, the changes observed were similar across the different vaccine types.

A health care worker wearing blue rubber gloves, a face mask and protective clothing administers a shot to the upper arm of a young woman.
A health care worker administers a COVID-19 vaccine to a young woman during a vaccination event in Miami in 2021. (Chandan Khanna/AFP via Getty Images)

Christine Metz, a professor at the Feinstein Institutes for Medical Research at Northwell Health, who wasn’t involved in the research, told Yahoo News that experiencing some level of variation in the menstrual cycle is normal.

“Lots of things can change the menstrual cycle, and it’s typically things that are going to affect the reproductive hormones because that’s what actually controls the menstrual cycle,” Metz said. “Things like medications ... anything that causes hormone imbalances — stress, diet, travel, infections — could change the menstrual cycle.”

Although it isn’t clear yet why the COVID-19 vaccines bring about these minor menstrual cycle changes, Metz said the immune system’s response to vaccination could influence reproductive hormone patterns enough to alter the menstrual cycle.

“The hormones that regulate the shedding of the menstrual cycle are influenced by our immune system,” she said. “People who received the vaccine, if they were sick from the vaccine, which many people were, you know, stayed in bed for a day because they had fever, they changed their diet, obviously, if they weren’t well. Those could all be factors that could influence the [menstrual] cycle.”

Edelman said another explanation is that our body’s immune and reproductive systems are interconnected and “talk to each other.”

“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 disturbance in the processes around menstruation, which results in some individuals experiencing changes,” she explained.

Although Metz praised the study for providing valuable information on the possible effects of COVID vaccines on menstrual health, she also said the study had some limitations. One is that it didn’t include women who were taking birth control pills, or those who already experienced irregular cycles prior to vaccination. Teens, Metz said, were also not included in the research. In addition, she said, “an overwhelming majority” of participants were white.

“We don’t know what the effect is on minorities, really, because the majority of the people in the study said they were white, so that’s kind of a problem. It would be great if we had more diverse populations,” she said.

Both experts noted that clinical trials of COVID vaccines didn’t look for effects on the menstrual cycle. Metz said this important marker of women’s health and fertility is often ignored.

“I think that’s just one of the side effects of being female. They don’t ask people about their changes in menstrual status. So we don’t know, and we don’t know the effects of other vaccines on menstrual cycle changes either,” she said, adding that a lack of knowledge isn’t helpful because it can lead to unfounded conspiracy theories, as well as vaccine hesitancy.

But Edelman hopes this new research can bring more awareness to the issue and motivate vaccine developers to collect this important information in vaccine trials moving forward.

“The public have spoken," she said. "This is important to them, and that should not be surprising. Half the population will, does and has menstruated. It’s a routine biological function that has meaning both to the individual and to science. I hope we can learn from this for ongoing and future vaccine trials.”