Don’t Panic If COVID Vaccine Slightly Alters Your Menstrual Cycle, Study Finds

Women vaccinated against COVID-19 saw their period slightly delayed by almost a day compared to those who were not vaccinated, a study funded by the US government said Thursday.

But the number of bleeding days was not affected, according to research conducted on nearly 4,000 individuals and published in Obstetrics and Gynecology.

Lead author Alison Edelman of Oregon Health & Science University told AFP the effects are small and should be temporary, a finding “very reassuring” and valid for those who have experienced the changes.

The study may also help counter anti-vaccine misinformation on the subject, which is rampant on social media.

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

Menstrual cycles typically last around 28 days, but the precise amount varies from woman to woman, as well as over an individual’s lifetime. It can also change during times of stress.

For their study, the scientists analyzed anonymized data from a fertility tracking application in women aged 18 to 45 who were not using hormonal contraception.

Some 2,400 participants were vaccinated – the majority with Pfizer (55%), followed by Moderna (35%) and Johnson & Johnson (7%).

About 1,500 unvaccinated women were also included for comparison.

Among the vaccinated group, data were collected from three consecutive cycles before vaccination and from three consecutive additional cycles, including the cycle (s) in which vaccination took place.

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

Immune response to vaccine

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

The response of the immune system to the vaccine may be the cause of the change.

“We know the immune system and the reproductive system are linked,” Edelman said.

A Boosted Immune System Could Impact the Hypothalamic-Pituitary-Ovarian Axis – What Edelman calls “the highway of how your brain talks to your ovaries, your uterus” or just “the body clock. “.

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

The changes seem more pronounced when the vaccination takes place at the start of the follicular phase, which begins on the first day of the menstrual period (bleeding) and ends at the onset of ovulation.

In fact, a subgroup of people who received two injections of the Pfizer or Moderna vaccine in the same cycle, as opposed to two different cycles, saw an average increase in cycle length of two days – but the effect is showing again temporary.

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

© Agence France-Presse


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