vaccination can alter the menstrual cycle

Several studies show a slight lengthening of the menstrual cycle or more abundant blood flow in some women of childbearing age after being vaccinated against COVID-19.

Nevertheless, these abundant flows are temporary, since everything returns to normal during the following cycle or two. In an article published in Science, researcher at Imperial College London, UK, Victoria Male, rectifies the rumors and provides an update on the possible effects of the anti-COVID vaccination on the menstrual cycle.

“Given that menstrual cycles are subject to natural variation, determining whether reported disruptions were attributable to COVID-19 vaccination or background variation was a particular challenge,” explains the researcher. « This highlights 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. »

In different studies, the type of vaccine received did not influence the risk of experiencing these changes, which, according to the researcher, suggests that these disturbances are likely 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.

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.

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.

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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