Anti-Covid vaccine and menstrual cycle alteration: what we know

Several studies have shown that the menstrual cycle can be temporarily altered following the administration of the vaccine against Covid 19. In fact, at the end of October, the EMA’s Pharmacovigilance Risk Assessment Committee (Prac) decided to add the increase in the duration or volume of bleeding among symptoms associated with vaccination against Sars-Cov-2. The Italian Medicines Agency (Aifa), however, has not yet reported any official reports in this regard. In the report, however, it is emphasized that the alterations of the menstrual cycle are transient and do not risk affecting fertility or reproduction.

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Stress, vaccines and the menstrual cycle

Stress is one of the most frequent causes of delay or alteration of the normal menstrual cycle. Whether it is caused by work, by intense physical training – and therefore by fatigue -, by weight loss, or by personal, family or emotional reasons, it is often the hormonal balance and, with it, the cycle that pays the price. menstrual. And among the possible stressors for the body, those involving the immune system, such as an infection or even the administration of a vaccine, should not be forgotten. However, although about half of the population vaccinated against any disease is female, there are few studies that have investigated the correlation between vaccines and the menstrual cycle. The arrival of the first vaccines against Covid 19 definitively opened the doors to this discussion. With the risk that the fear of consequences on the menstrual cycle and fertility would increase vaccination hesitancy.

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What the studies say

Since the vaccination campaign began, statistical surveys have confirmed a correlation between alteration of the menstrual cycle and administration of the vaccine. The disturbances reported by women are essentially two: a delay in the arrival of menstruation, and therefore a lengthening of the cycle, or an increase in bleeding during menstruation. Since the menstrual cycle, as mentioned, naturally varies as a result of several factors, the main difficulty of these studies has been to determine the extent to which the reported changes can be attributed to vaccination, in this case against Covid-19, rather than to other changes.

A first US study analyzed the problem using data recorded by the Natural Cycles cycle monitoring app, on a sample of 4,000 women residing in the United States. The analysis was then extended to a global sample of 20,000 women, 14,000 of whom were vaccinated and the rest were not vaccinated. The results showed that the women recorded an extension of the cycle of less than one day on average, with an increase to a few days if the first and second doses were administered within the same cycle. In both studies, however, the women reported that the situation returned to normal within two cycles. A similar conclusion was also reached by a study conducted on nearly 10,000 women, also residing in the United States, as part of the Apple Women’s Health Study, based on data collected from an application of Apple devices.

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Last month, the first Italian pilot study was also published on a sample of 100 women aged between 18 and 45 who had received at least two doses of the vaccine. The results show that menstrual delays and increased bleeding are quite common symptoms in the three weeks following vaccination, especially after the second injection. In the study, which however is not conclusive and needs to be expanded, the authors emphasize that these are always transitional changes that do not risk altering women’s fertility.

Two possible mechanisms at play

It is therefore possible that a vaccine – not just the one against Covid 19 – could induce a state of stress in the body and cause irregularities in the menstrual cycle. However, there are only hypotheses about what specific interferences the vaccine against Sars-Cov-2 induces in the body. As for the lengthening of the menstrual cycle, one possible explanation is that the immune response induced by the vaccine directly interferes with sex hormones. A confirmation could come from the fact that those who take hormonal contraceptives have reported fewer changes statistically. However, there would be at least one other factor at play, given that the lengthening of the menstrual cycle also seems to depend on the moment of the cycle in which the vaccine is administered: in particular, the duration of the cycle is expected to increase if the woman is vaccinated during the first phase, the follicular phase preceding ovulation. A hypothesis supported, for now, by the results of the Apple Women’s Health Study.

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In the case of increased bleeding, however, the hypothesis is that it is due to a decrease in the repair capacity of the endometrial tissue in the last phase of the cycle, the luteal one. In this case, the vaccine would act directly on the functionality of the cells of the immune system responsible for tissue repair at the uterine level. The studies conducted have also shown that this alteration is more evident as women age and is therefore also associated with the body’s ability to regenerate cells.

None of these hypotheses, however, can be said to be confirmed and more extensive and dedicated studies are needed to understand the mechanisms at play. What is clear, however, is that the alteration of the menstrual cycle is due to the immune system’s response to the vaccine, and not to a specific component of the vaccine. The results of the investigations, in fact, do not change if we consider a specific type of vaccine (mRna or viral vector). And above all, that all these changes do not risk compromising women’s sexual health or fertility, and return after a few weeks. According to experts, this research would be an opportunity to be seized in the development of future vaccines, as rarely when studying the side effects of a vaccine, one wonders whether these may also concern the menstrual cycle or fertility.

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