COVID-19 vax tied to small, transient change in menstrual cycle length

COVID-19 vaccination appears to be associated with a small increase in menstrual cycle length of less than 1 day, but these changes are only temporary and there is no change in the duration of menses, a prospective study suggests.  

“Our findings are reassuring,” said the researchers, who explained that “Concerns about a possible association between COVID-19 vaccination and abnormal menstrual cycles may lead to vaccine hesitancy.”

While there have been anecdotes on social media suggesting menstrual disturbances following COVID-19 vaccination, the researchers noted that a lack of evidence “limits our ability to sufficiently address these concerns and to counsel individuals who menstruate about what to expect with vaccination.”

The prospective study tracked menstrual cycle data of 3,959 individuals aged 18–45 years in the US with normal cycle lengths using the “Natural Cycles” application. Among the participants, 2,403 were vaccinated (55 percent with Pfizer-BioNTech vaccine and 35 percent with Moderna vaccine) while 1,556 were unvaccinated. Their menstrual data were collected for six consecutive cycles, starting from three cycles before the first vaccine dose for vaccinated participants. [Obstet Gynecol 2022;doi:10.1097/AOG.0000000000004695]

Compared with the cycles before vaccination, COVID-19 vaccination was associated with an increase of less than 1 day in the cycle length, regardless of whether it was following the first dose (0.71 day increase, 98.75 percent confidence interval [CI], 0.47–0.94) or the second dose (0.91 day increase, 98.75 percent CI, 0.63–1.19).

On the other hand, unvaccinated participants did not experience any significant change in cycles four and five compared with the first three baseline cycles (0.07 and 0.12 day for cycles four and five, respectively).

Although the difference in cycle length was statistically significant between the vaccinated vs the unvaccinated cohort, the increase in cycle length of less than 1 day, according to the researchers, was not clinically significant.

In a subset of participants who received both vaccine doses during the same menstrual cycle, their cycle length increased by about 2 days compared with those who were unvaccinated (2.38 days, 98.75 percent CI, 1.52–3.24).  

Moreover, a greater proportion of vaccinated participants saw their menstrual cycle length increased by ≥8 days compared with unvaccinated individuals (10.6 percent vs 4.3 percent; p<0.001).

“Although approximately 10 percent of these individuals experienced a clinically notable change in cycle length of 8 days or more, this change attenuated quickly within two post-vaccine cycles,” they observed. This indicates that the changes in menstrual cycle length were likely temporary.

“mRNA vaccines create a robust immune response or stressor, which could temporarily affect the hypothalamic-pituitary-ovarian axis, if timed correctly,” the researchers explained. The hypothalamic-pituitary-ovarian axis is involved in regulation of the menstrual cycle.

By contrast, there was no change in the duration of menstrual bleeding in both the vaccinated and unvaccinated cohorts.

“Menstrual cyclicity is an overt sign of health and fertility,” the researchers pointed out.

According to guidance from The International Federation of Gynecology and Obstetrics, a variation of less than 8 days in cycle length is considered as normal.

“We find no population-level clinically meaningful change in menstrual cycle length associated with COVID-19 vaccination. Our findings support and help explain the self-reports of changes in cycle length,” the researchers concluded.

“Questions remain about other possible changes in menstrual cycles, such as menstrual symptoms, unscheduled bleeding, and changes in the quality and quantity of menstrual bleeding,” they added.