For many months now, some women have complained about seeing their periods disturbed after a vaccination against Covid.
An American study published in the journal Obstetrics & Gynecology on Thursday returns to the subject, specifying that the menstrual cycle, the period between the first day of two periods, can be lengthened by less than a day on average, a non-serious effect. and which appears to be temporary.
On the other hand, the duration of the rules itself is not affected by the vaccination, according to this work carried out 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 their cycle 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," said Alison Edelman, lead author of the study and professor of obstetrics and gynecology at Oregon Health & Science University.
"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.
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 data filled in by women aged 18 to 45, and not using contraception, on an application used to monitor their cycles and 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%).
Finally, 1,500 unvaccinated people were also included in the study as a point of comparison.
A slightly disturbed "body clock"
Six consecutive cycles were studied for all participants, but for the first group, an injection of vaccine was received during the fourth cycle.
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).
How to explain this change?
“We know that the immune and reproductive systems are interconnected,” says Alison Edelman.
However, vaccines create a strong immune response.
This response affects the hypothalamic pituitary-ovarian axis, which the specialist describes as “the communication highway between the brain, the ovaries and the uterus”.
Read also Anti-Covid vaccines: should we be concerned about a possible link with menstrual disorders?
This axis helps regulate the menstrual cycle, which is why the researcher also gives it the nickname “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 seems to be more pronounced when the vaccination is carried out "early in the follicular phase" (starting on the first day of menstruation and going until the moment of ovulation).
Scientists are also collecting data globally, so that they can differentiate the effects of several vaccines.
In France, menstrual disorders after vaccination had been placed under surveillance by the Medicines Agency (ANSM).
Last December, however, the body had indicated that "no direct link between the vaccine and the occurrence of cycle disorders" had not been highlighted by the data collected.