Women vaccinated against Covid-19 experienced a slight delay, almost a day, in their menstrual cycle, compared to unvaccinated women, notes a study funded by the US government and published on Thursday.
The number of days of bleeding, however, was not affected, concluded the survey, conducted with nearly 4,000 people and published in “Obstetrics & Gynecology”. Lead author Alison Edelman of Oregon Health Sciences University told AFP the effects are small and should be temporary.
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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.
Scientists in the study analyzed anonymous data from a fertility tracking app among women ages 18 to 45 who were not using hormonal contraceptives. Two thousand four hundred participants were vaccinated, the majority with Pfizer (55%), followed by Moderna (35%) and Johnson & Johnson (7%). Approximately 1,500 unvaccinated women were included for comparison.
Among the vaccinated group, data were collected from three consecutive menstrual cycles before vaccination and from a further three consecutive cycles, including the cycle(s) in which vaccination occurred. For unvaccinated women, data were collected for six consecutive cycles.
– Immune response to the vaccine –
On average, the first dose of the vaccine was associated with a 0.64 day increase in menstrual cycle length, and the second dose with an increase of 0.79 days when comparing the vaccinated group with the unvaccinated group. The immune system’s response to the vaccine may be behind the change.
“We know that the immune system and the reproductive system are related,” explained Alison. The changes seem more pronounced when vaccination is done at the beginning of the follicular phase, which starts on the first day of the menstrual period (bleeding) and ends at the beginning of ovulation.
A subset of women who received two doses of Pfizer or Moderna in the same cycle, rather than two different cycles, saw an average increase in cycle length of two days, but the effect again appears to be temporary.
The team hopes to gather more data on subsequent cycles among vaccinated women, to confirm a long-term return to baseline, and to expand the study globally to differentiate effects across vaccine brands.
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