Menstrual cycle lengthened by less than a day after Covid vaccine, study finds

Right after receiving a vaccine against Covid-19, the women’s menstrual cycle is extended by less than a day on average, according to a new study published Thursday, January 6. The length of periods itself is not affected by vaccination, according to research conducted in the United States on nearly 4,000 women.

This study should in particular make it possible to reassure those who have observed changes in the length of the period between the first day of two periods after a vaccine injection. It will also make it possible to oppose clear and solid data – the first on the issue – to the fears and false claims that have circulated on social networks.

“Very reassuring” results

The results “Are very reassuring”Alison Edelman, lead author of the study and professor of obstetrics and gynecology at Oregon Health & Science University, told AFP.

“We do not find any clinically significant change in the length of the menstrual cycle associated with vaccination against Covid-19, also poses the study, funded by the National Institutes of Health (NIH) and published in the journal Obstetrics & Gynecology.

Any change lasting less than eight days is classified as normal by the International Federation of Gynecology and Obstetrics, she recalls.

If a cycle generally lasts about 28 days, this duration varies from one woman to another, but also in a woman during her life. The duration can for example change during times of stress.

For their work, the scientists analyzed the data filled in by women aged 18 to 45, and not using contraception, on an application used to monitor their cycles (for example to know their fertility periods), validated by the United States Medicines Agency.

They looked at the cycle length of some 2,400 people vaccinated – mostly with Pfizer (55%), but also with Moderna (35%) and Johnson & Johnson (7%). 1,500 unvaccinated people were also included in the study as a point of comparison. Six consecutive cycles were studied for all participants, but for the first group, a vaccine injection was received during the fourth cycle.

The cycle lengthens by less than a day

Results: between the first three cycles and the fourth, an increase in duration was indeed observed in the vaccinated group, but of less than one day (0.64 days).

For the cycle in which the second dose was received (the fifth for the majority of participants), the increase was somewhat more pronounced, but still less than a day on average (0.79).

What is this change due to?

“We know that the immune and reproductive systems are interconnected”, explains Alison Edelman. However, vaccines create a strong immune response. This response affects the hypothalamic pituitary-ovarian axis, which the specialist describes as “The highway of communication between the brain, the ovaries and the uterus”. This axis helps to regulate the menstrual cycle, which is why the researcher also gives it the nickname of“Body clock”.

With vaccination, “You release proteins called cytokines, which we know from other diseases can disrupt this body clock”, she explains. The change also appears to be more pronounced when the vaccination is performed. “Early in the follicular phase” (starting on the first day of menstruation and continuing until ovulation).

Indeed, by isolating the people who received the two injections of Pfizer or Moderna during their fourth cycle – and not on two different cycles -, the increase in the duration of the cycle was this time of two days.

“Individuals who have received two doses of the Covid-19 vaccine within the same cycle seem to experience a longer variation in the length of their cycle, but temporary”, detail the authors of the study. In these people, by the sixth cycle, the duration was again more or less the same as in the first three (the change was only 0.17 days).

Scientists hope to have gathered more data on subsequent cycles very soon to confirm this return to normal. They also collect data globally, so that they can differentiate the effects of multiple vaccines.


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