Covid-19 vaccines cause heavier menses, study shows

A health worker administers Covid-19 vaccine to a church member at Mombasa Pentecostal Church in Bamburi in April this year.

Photo credit: File | Nation Media Group

Women who received the Covid-19 vaccine experienced heavier bleeding during their periods, a study has revealed. Researchers have found that almost half of trial participants who were menstruating regularly had heavy flow after the vaccine.

They also established that those who did not typically menstruate like people on long-acting contraceptives and post-menopausal women experienced unusual bleeding. This is contained in one of the largest peer-reviewed studies to date on the effects of Covid-19 vaccines on menstruation. It was carried out by researchers at the University of Illinois at Urbana-Champaign and Washington University School of Medicine in St Louis. They distributed an online survey in April last year to thousands of people around the world.

Respondents aged 18–80

The study is a product of more than 39,000 responses collected and analysed from individuals aged 18–80, with the researchers assuring that all the respondents had been fully jabbed with Moderna, Pfizer-BioNTech and Johnson & Johnson vaccines and had not contracted the coronavirus before being vaccinated, at least to the best of their knowledge.

The findings, published in the Journal Science Advances, show that 42 per cent of people with regular menstrual cycles experienced heavier bleeding after vaccination, while 44 per cent reported no change and 14 per cent reported lighter periods.

The researchers further disclose that 39 per cent of respondents were on gender-affirming hormone treatments, 71 per cent of people on long-acting contraceptives and 66 per cent of postmenopausal women experienced breakthrough bleeding after one or both of vaccine shots.

Vital findings

According to Katharine Lee, a biological anthropologist at the Washington University School of Medicine in St Louis, and the study’s first author, their findings are very vital.

“It’s important that people know this can happen so that they’re not scared, they’re not shocked and they’re not caught without supplies,” she said, pointing out that the study did not compare the results with a control group of people who did not get vaccinated and that it is possible people who observed changes in their cycles after vaccination were more likely to participate in the survey.

He noted that their findings showed some demographics may be more likely to experience menstrual changes, hence what they have established is very important in helping them prepare.

“A heavier menstrual flow was more likely for those who were older, for instance. Respondents who used hormonal contraception, had been pregnant in the past, or had been diagnosed with a reproductive condition like polycystic ovarian syndrome, endometriosis or fibroids were also more likely to have heavier bleeding during their periods.”

Pointing out that postmenopausal women who were slightly younger averagely aged around 60 were more likely to experience breakthrough bleeding after the vaccine than those who were older, the experts found that the type of vaccine postmenopausal women received, whether they had other side effects like fever or past pregnancy, did not seem to have an effect on their bleeding.

Last year, more than 85 women told the Nation that their menstrual flow either came way earlier than expected or had yet to show up about three weeks after they received the AstraZeneca jab.

“My periods came a week earlier than usual and it happened after I was immunised for coronavirus. I have not been stressed lately and neither have I engaged in any sexual activity for quite some time now," an entrepreneur based in Nairobi said, disclosing that the quantity of the flow – when it arrived – was much less than usual and much more dilute as well.

Different reactions

Dr Nelly Yatich, a Kenyan epidemiologist, explained that different people react differently to medication. "There are some medications that can alter one's cycle, but I don't know if this is a normal reaction as it is not a documented side effect."

In an official response to the Nation, the Pharmacy and Poisons Board (PPB) urged all women to report their experiences to them officially. "We are committed to ensuring all vaccines offered in the country are as safe as possible.  In that regard, the Ministry of Health encourages all members of the public to report any unusual incidents after receiving the Covid-19 vaccine to the Pharmacovigilance Electronic Reporting System via," the regulator said in an email response.

Experts have in the past explained that hormones secreted by the hypothalamus, the pituitary gland and the ovaries regulate the monthly cycle, and they can be affected by both internal and external factors such as stress and illness, weight loss or gain, calorie restriction and intense exercise.

In a past study on the same subject, Dr Alison Edelman, a professor of obstetrics and gynaecology at Oregon Health & Science University, explains that the endometrium, which lines the uterus and is shed during menstruation, is linked to the immune system because of the role it plays in the remodelling of uterine tissue and protection against pathogens.

This, according to the professor, is the reason it is possible that when vaccines activate the immune system, they somehow trigger downstream effects in the endometrium, thus causing a disturbance in a woman’s menstrual cycle.


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