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COVID-19 vaccination and menstrual cycle – global cohort study

COVID-19 vaccination is associated with a small and likely temporary change in menstrual cycle length but with no change in menses length, nor any discernible effect on fertility, according to a large study in the British Medical Journal.

When, shortly after the rollout of coronavirus vaccines last year, women began posting on social media about changes to their periods, their claims weren’t taken terribly seriously, but recent research shows that many of the complaints were valid, writes The Washington Post. The study of nearly 20 000 women worldwide shows that a COVID shot can, in fact, change the timing of the menstrual cycle. Vaccinated women experienced, on average, about a one-day delay in getting their periods, compared with those who hadn’t been vaccinated,

The data for the study were taken from a popular period-tracking app called Natural Cycles and included people from around the world, but most were from North America, Britain and Europe. The researchers used “de-identified” data from the app to compare menstrual cycles among 14 936 participants who were vaccinated and 4 686 who were not.

Because app users tracked their menstrual cycles each month, the researchers were able to analyse three menstrual cycles before vaccination and at least one cycle after, and compare them with four menstrual cycles in the unvaccinated group.

The data showed that vaccinated women got their periods 0.71 days late, on average, after the first dose of vaccine. However, those who received two vaccinations within one menstrual cycle experienced greater disruptions. In this group, the average increase in cycle length was four days, and 13% experienced a delay of eight days or more, compared with 5% in the control group.

Alison Edelman, a professor of obstetrics and gynaecology at Oregon Health & Science University, who led the study, said that for most people the effects were temporary, lasting for one cycle before returning to normal. She said there were no indications that the period side effects had any impact on fertility.

“Now we can give people information about possibly what to expect with menstrual cycles,” Edelman said. “So I hope that’s overall really reassuring to individuals.”
Researchers don’t know exactly why the vaccines seem to affect menstrual cycles, but Edelman said that the immune and reproductive systems are linked and that inflammation or a strong immune response could trigger menstrual fluctuation.

Any change in getting your period can be stressful, and women have expressed frustration that public health officials didn’t warn them about the possible side effect or do more research before rolling out the vaccines. One major limitation of the study is the fact that it included only those who were not on birth control, had regular cycles before getting vaccinated, and were aged 18 to 45.

The study also didn’t answer all of the questions raised about vaccines and periods, including how the shots affect trans men and nonbinary individuals.

This study did not look at the heaviness of periods or other side effects such as cramps, but researchers said it did show that, on average, getting vaccinated did not appear to cause longer periods.

Edelman said preliminary findings from a different study suggest that the vaccine sometimes causes heavier periods. The data, collected from nearly 10 000 people, are still undergoing peer review, but showed that getting vaccinated increased slightly the probability of heavier bleeding.

However, she acknowledged that her studies have looked only at women with normal menstrual cycles who aren’t using hormonal contraceptives, and that individual experiences may vary widely.

Caiityya Pillai, 21, who lives in Berkeley, said that for two months after her March 2021 shot, her normally light period became extremely painful and lasted twice as long.

“The pain wasn’t like a normal pain. I was crying and could not get out of bed,” she said.

Pillai said she was overwhelmed with anxiety and thought something else might be wrong, but that after two cycles, her period returned to normal. When she got a second dose in July 2021, her period worsened again, but she said she felt calmer about it because she had seen similar stories being shared online.

Other research has suggested that the vaccines have a variety of effects on periods.

Another survey collected information about periods and vaccines from 160,000 people – including transgender and postmenopausal people – and found that thousands reported heavier bleeding than usual or breakthrough bleeding.

While these observations aren’t necessarily medically alarming, Katharine Lee, an assistant professor at Tulane University who led the survey, said the information is important to help trans men plan for additional support if menstruating causes gender dysphoria, and also to help people make decisions about stocking up on
tampons and pads.

Lorena Grundy, 27, uses an IUD and hadn’t had her period for over three years before she got her first Pfizer shot in February 2021. The next day at work, she got her period.

“It wasn’t that the vaccine moved my period early or late: it produced one,” she said.

Her period lasted three or four days – and it came back when she got her second vaccine dose three weeks later. But it didn’t happen again when she got a booster shot last November.

Diana Bianchi, the director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, which funded Edelman’s research, said getting a significantly late period after vaccination is not necessarily cause for alarm.

“I wouldn’t recommend going to a doctor after the first time that it happens, just because all the evidence indicates that the change resolves, it’s only temporary,” she said. “If it’s a persistent change in the menstrual cycle interval, then that might be a reason to see your primary-care physician or OB/GYN.”

The National Institutes of Health has funded at least four other research projects around coronavirus vaccines and menstruation – some of which look at adolescents and people with endometriosis º with the hope of providing better information and increasing public trust in the vaccines.

Study details

Association between menstrual cycle length and covid-19 vaccination: global, retrospective cohort study of prospectively collected data

Alison Edelman, Emily R Boniface, Victoria Male, Sharon T Cameron, Eleonora Benhar, Leo Han, Kristen A Matteson, Agathe Van Lamsweerde, Jack T Pearson and Blair G Darney.

Published in The BMJ on 27 September 2022


To identify whether COVID-19 vaccines are associated with menstrual changes to address concerns about menstrual cycle disruptions after COVID-19 vaccination.

International users of the menstrual cycle tracking application, Natural Cycles.

19 622 individuals aged 18-45 years with cycle lengths of 24-38 days and consecutive data for at least three cycles before and one cycle after COVID (vaccinated group; n=14 936), and those with at least four consecutive cycles over a similar time period (unvaccinated group; n=4686).

Main outcome measures
The mean change within individuals was assessed by vaccination group for cycle and menses length (mean of three cycles before vaccination to the cycles after first and second dose of vaccine and the subsequent cycle). Mixed effects models were used to estimate the adjusted difference in change in cycle and menses length between the vaccinated and unvaccinated.

Most people (n=15 713; 80.08%) were younger than 35 years, from the UK (n=6222; 31.71%), US and Canada (28.59%), or Europe (33.55%). Two thirds (9929 (66.48%) of 14 936) of the vaccinated cohort received the Pfizer-BioNTech (BNT162b2) covid-19 vaccine, 17.46% (n=2608) received Moderna (mRNA-1273), 9.06% (n=1353) received Oxford-AstraZeneca (ChAdOx1 nCoV-19), and 1.89% (n=283) received Johnson & Johnson (Ad26.COV2.S). Individuals who were vaccinated had a less than one day adjusted increase in the length of their first and second vaccine cycles, compared with individuals who were not vaccinated (0.71 day increase (99.3% confidence interval 0.47 to 0.96) for first dose; 0.56 day increase (0.28 to 0.84) for second dose). The adjusted difference was larger in people who received two doses in a cycle (3.70 days increase (2.98 to 4.42)). One cycle after vaccination, cycle length was similar to before the vaccine in individuals who received one dose per cycle (0.02 day change (99.3% confidence interval −0.10 to 0.14), but not yet for individuals who received two doses per cycle (0.85 day change (99.3% confidence interval 0.24 to 1.46)) compared with unvaccinated individuals. Changes in cycle length did not differ by the vaccine’s mechanism of action (mRNA, adenovirus vector, or inactivated virus). Menses length was unaffected by vaccination.

COVID-19 vaccination is associated with a small and likely to be temporary change in menstrual cycle length but no change in menses length.


The BMJ article – Association between menstrual cycle length and covid-19 vaccination: global, retrospective cohort study of prospectively collected data (Creative Commons Licence)


The Washington Post article – Women said coronavirus shots affect periods. New study shows they’re right (Restricted access)


See more from MedicalBrief archives:


UK reports 30,000 “yellow card” menstrual-related vaccination occurrences


Research turns to potential affect of COVID vaccines on menstrual cycle


Unpacking COVID vaccination's effect on fertility and sexual functioning




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