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(CNN Spanish) -
After receiving a dose of the vaccine against covid-19, there were women who experienced an increase in the duration of the menstrual cycle of approximately one day more than usual on average, according to a study funded by the National Institutes of Health (NIH) that was published this Thursday.
However, the change is not clinically significant, according to the researchers.
The researchers note that the average increase in menstrual cycle length "appears to be largely driven" by women who received two doses of an mRNA vaccine, ie Pfizer / BioNTech or Moderna, within the same menstrual cycle. .
In this group, cycle length increased by an average of about two days.
However, the change was temporary and resolved within a couple of months, according to the study.
There is not enough data to say how long the change lasted for the other women.
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Overall, about 5% of vaccinated women had a clinically significant change in their cycle of more than eight days, but the rate was about the same among unvaccinated women.
Neither vaccinated nor unvaccinated women in the study had a specific change in the length of their period.
The researchers analyzed data from about 4,000 women who used the Natural Cycles app to track menstruation, including about 2,400 who got vaccinated over the course of the study and about 1,600 who didn't.
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Regulation of the menstrual cycle can be affected by daily life, the environment, and stress-related health factors.
The researchers ruled out pandemic-related stress as the cause of the change in cycle length.
However, they note that mRNA vaccines create a similar robust immune response that could temporarily affect the regulation of the menstrual cycle.
An acute serious illness, such as COVID-19, could be "catastrophic" for this regulation, sometimes permanently, they wrote.
More research on the impact of the vaccine on the menstrual cycle
In August, NIH invested about $ 1.7 million to support five research teams studying the possible effects of vaccines on menstruation.
This is the first of those studies to be published.
"The study, on the one hand, validates what some women were saying on social media," Dr. Diana Bianchi, director of the NIH Eunice Kennedy Shriver National Institute of Child Health and Human Development, told CNN.
She was not directly involved in the investigation.
"But in general, at the health level of the population, this slight change does not really have clinical importance. It should not affect fertility, and the benefits of being vaccinated and not contracting covid -even a slight version of the omicron variant- are much older. You really do not have to hesitate to get vaccinated, "he said.
This study does not specifically address fertility or other possible changes in menstrual cycles, such as unforeseen symptoms or bleeding.
The researchers looked at only data from women who typically have regular menstrual cycles, but noted that many do not fall into this category.