Study: COVID shots alter women's menstrual cycles; experts say effect is 'small,' 'temporary'

Coronavirus vaccines can alter a woman's menstrual cycle, a new study asserts, though its authors claim that the effect is both small, temporary, and not outside the normal bounds of cycle variation. 

The study, published in BMJ Medicine, claims that "COVID-19 vaccination" is associated with "an average increase in menstrual cycle length of less than one day," the National Institutes of Health said in a press release. The NIH helped fund the study.

"The increase was not associated with any change in the number of days of menses," the NIH claimed, and "for most study participants, the increase resolved in the cycle following vaccination."

“These findings provide additional information for counseling women on what to expect after vaccination,” Diana Bianchi, the director of the NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development, said in the release.

"Changes following vaccination appear to be small, within the normal range of variation, and temporary," she added. 

"A change in cycle length of less than eight days is considered within the normal range of variation," the NIH noted in the press release.

Reports have circulated following the release of the vaccines of women experiencing altered cycles and periods, though hard data have been elusive on the matter. 

An earlier NIH-backed study came to the same conclusion regarding COVID vaccines and menstrual cycles.