Menstrual cycle may be slightly delayed after coronavirus vaccine, study finds

Women vaccinated against COVID-19 experienced a slight delay in their menstrual period nearly a day compared to those who weren’t vaccinated, a US government-funded study published Thursday.

However, the number of days of bleeding was not affected, according to research in almost 4,000 people and published in “Obstetrics & Gynecology”.

The main author, Alison Edelman, from the Oregon Health and Sciences University, told AFP that the effects are small and expected to be temporary, a finding “Very reassuring” and validated by those who experienced changes.

The study may also help counter vaccine misinformation regarding the menstrual period, which is widely circulated on social media.

The slight increase in the duration of the menstrual cycle it is not clinically significant. Any change of less than eight days is classified as normal by the International Federation of Gynecology and Obstetrics.

The menstrual cycles They generally last around 28 days, but the exact amount varies from woman to woman, as well as over a person’s lifetime. It can also change during times of stress.

For their study, the scientists analyzed anonymous data from a fertility tracker app among women ages 18 to 45 who were not using hormonal contraceptives.

About 2,400 participants were vaccinated, most with Pfizer (55%), followed by Modern (35%) and Johnson & Johnson (7%). Some 1,500 unvaccinated women were also included for comparison.

Among the vaccinated group, data were collected from three consecutive menstrual cycles prior to vaccination and from three more consecutive cycles, including the cycle (s) in which vaccination occurred.

For unvaccinated women, data was collected for six consecutive cycles.

Immune response to the vaccine

On average, the first dose of vaccine was associated with a 0.64-day increase in the duration of menstrual cycle and the second dose with an increase of 0.79 days, when comparing the vaccinated group with the unvaccinated one.

The immune system’s response to the vaccine could be behind the change.

“We know that the immune system and the reproductive system are interrelated”, Edelman explained.

An accelerated immune system could have an impact on the hypothalamic-pituitary-ovarian axis, what Edelman calls a “Highway where the brain talks with the ovaries, with its uterus” or just the “biological clock”.

Specifically, the production of inflammatory proteins called cytokines appears to alter the way this axis regulates the timing of menstrual cycles.

The changes appear more pronounced when vaccination is done early in the follicular phase, which begins on the first day of the menstrual period (bleeding) and ends when ovulation begins.

In fact, a female of people who received two injections of the vaccines Pfizer or Modern During the same cycle, as opposed to two different cycles, they saw an average increase in cycle length of two days, but the effect again seems temporary.

The team now hopes to collect more data on subsequent cycles among vaccinated women to confirm a long-term return to baseline and expand the study globally so that they can differentiate the effects between vaccine brands.