Vaccination against COVID-19 can alter the menstrual cycle

The fact that some women of childbearing age had their menstrual cycles disrupted after being vaccinated against COVID-19 worried many, who were then hesitant to continue the vaccination program. Although there is overwhelming evidence confirming that vaccination does not affect fertility, a stream of misinformation to the contrary has unfortunately swept through reports of heavy bleeding and prolonged menstrual cycles.

In an article published in ScienceVictoria Male, a researcher in the Department of Metabolism, Digestion and Reproduction at Imperial College London, UK, rectifies rumors and provides an update by citing studies that have looked at the possible effects of COVID vaccination on periods.

As of April 2022, the system collecting reports of adverse reactions likely related to the COVID-19 vaccine in the United States had received 11,000 reports of menstrual changes and unexpected vaginal bleeding after vaccination. The British equivalent had collected 50,000.

“Given that menstrual cycles are subject to natural variation, determining whether reported disruptions were attributable to COVID-19 vaccination or background variation presented a particular challenge.This underscores the need for an analytical method that includes comparison with a control group of unvaccinated women to identify with more certainty any changes that may be related to vaccination,” says Victoria. Male.

The results obtained in the few studies carried out in good and due form highlight in some women a very slight lengthening of the menstrual cycle or a more abundant blood flow which are temporary, since everything returns to normal during the following cycle or two. .

In these different studies, the type of vaccine received did not influence the risk of undergoing these changes, which, according to the author, suggests that these disturbances are probably the result of the immune response induced by the vaccine. In support of this hypothesis: comparable menstrual changes were also observed following vaccination against typhoid, hepatitis B and human papillomavirus.

Immune mechanisms

Two immune mechanisms are proposed to explain these disturbances of the menstrual cycle. According to one of them, the first responses of the immune system to vaccination – which manifest themselves in the production of cytokines – could « interfere in the hormonal dialogue between the hypothalamus, the pituitary and the ovaries which regulates the menstrual cycle », which would contribute to the lengthening of the cycle. According to the second mechanism, the cytokines produced would affect the macrophages and the natural killer cells which control the regeneration of the endometrium, the internal lining of the uterus, which then thickens and fills with blood vessels in preparation for pregnancy. possible. And if the latter does not take place, this blood-soaked mucous membrane is evacuated during menstruation. This second mechanism could therefore explain the increase in blood flow.

The fact that women who take the ovarian hormones, estrogen or progesterone, for contraceptive purposes do not experience menstrual changes following their vaccination lends some credence to the hypothesis that these disturbances are of hormonal origin, worth Mme Male.

Another observation: the moment of the menstrual cycle when the person is vaccinated will determine whether there will be a lengthening of the cycle, or not. “The follicular phase of the cycle, which occurs before ovulation, can be prolonged by hormonal changes, while the luteal phase, which begins after ovulation, is of more constant duration. If menstrual changes are induced by immune effects on ovarian hormone control, then vaccination is expected to lengthen the follicular phase, and this will only occur if the vaccine is given during this same phase.” says the researcher. An American study involving nearly 10,000 women precisely indicates that only vaccination carried out during the follicular phase is associated with a lengthening of the cycle.

Additionally, a large survey of more than 27,000 women of menstruating age found that the older the women, the more likely they are to experience heavy bleeding during their period. A phenomenon which “seems to indicate impaired repair of uterine tissue which is coordinated by immune cells and which would be less effective in the elderly, i.e. the mechanism by which anti-COVID vaccination increases menstrual flow”, we underline in the article.

All that remains is to conduct studies aimed at measuring hormone levels before and after vaccination, as well as the immune cells present in endometrial biopsies or in menstrual fluid to confirm these two hypotheses.

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