Sept. 27 (UPI) -- A new study has confirmed the link between women who received a COVID-19 vaccination and a longer menstrual cycle, according to research published Tuesday in the journal BMJ Medicine.
The international study, funded by the National Institutes of Health, found the vaccinated group had a temporary change of cycle length of around 0.71 days after their first vaccine dose, researchers said confirming results from a previous U.S. study that linked COVID-19 vaccination with an average increase in menstrual cycle length of less than one day.
"Compared with the unvaccinated group, vaccinated individuals had an adjusted increase in menstrual cycle length of less than one day with both first and second vaccine doses," researchers from universities in Oregon, Massachusetts, London and Edinburgh wrote in the study. "Individuals who received two doses of a COVID-19 vaccine in a single cycle had an adjusted increase in cycle length of 3.70 days compared with the unvaccinated."
A change in cycle length of less than eight days is considered to be within the normal range of variation. While this temporary cycle variation would not alarm health care professionals, the change in a bodily function that controls fertility could contribute to vaccine hesitancy.
"These findings provide additional information for counseling women on what to expect after vaccination," said Dr. Diana Bianchi, director of NIH's Eunice Kennedy Shriver National Institute of Child Health and Human Development. "Changes following vaccination appear to be small, within the normal range of variation, and temporary."
Researchers analyzed data from 19,622 participants, of which 14,936 were vaccinated. Data from four consecutive cycles were studied. The type of vaccine, Moderna or Pfizer, did not change the results.
Almost 1,400 participants, or 6.2% of the vaccinated women, experienced a larger swing in their cycle length of eight or more days. The larger cycle changes were among younger women who tended to have longer cycle length before vaccination.
Tuesday's study comes after U.S. researchers conducted a study with more than 2,400 women in January that produced similar results. Researchers analyzed anonymous menstrual cycle data from a fertility tracking app called Natural Cycles.
The increase in cycle length, or longer time between bleeding, was not associated with any change in the number of days of menses, or bleeding, the data showed.
Clinical trials and subsequent monitoring of vaccine recipients conducted by the Centers for Disease Control and Prevention have found that side effects, aside from injection site pain and fatigue, are rare with COVID-19 vaccines.