A study out Thursday from Oregon Health and Science University found that while vaccines for COVID-19 did affect menstrual cycles for some people, that impact was minimal and generally resolved itself quickly, as soon as the next cycle.
Dr. Alison Edelman, the lead author of the study which was published Thursday in the journal Obstetrics & Gynecology, called the findings both reassuring to people who were concerned about the vaccine affecting their cycles and validating to those who experienced some changes.
“Just like we tell people, ‘You are going to get a headache with the vaccine,’ ‘You are going to get a low-grade fever for a couple days,’” Edelman, who works with OHSU’s OB/GYN and Family Planning group, said, “we can tell them, ‘You may have a slight change in your menstrual length.’”
Researchers used data compiled with user consent from the Natural Cycles app of nearly 4,000 vaccinated and unvaccinated people tracking their cycles in the United States. For the vaccinated, it took into account cycles both before and after vaccination doses.
The average change the study found was less than a one-day increase in length between periods in menstruating people.
Interestingly, the most profound change was in the small number of participants who had two shots during one cycle.
According to the study’s authors, “the increase in cycle length for both the first and second vaccine cycles appears to be driven largely by the 358 individuals who received both vaccine doses within a single cycle.”
For those people, the results showed an increase in cycle length of two days compared to unvaccinated people. Those variances in cycle length also resolved themselves quickly, the study found.
The study did not discover any change in the length of bleeding, or menses, in people after they were vaccinated.
According to Edelman, cycle length variation is normal and clinically, a cycle variation of fewer than eight days isn’t considered concerning.
Even if a patient presented with an eight-day or greater change in their cycle, Edelman said, clinicians would usually wait for at least three cycles to see if the change resolved itself.
The study of Natural Cycles users showed that among those people who received two shots within one cycle, “approximately 10% of these individuals experienced a clinically notable change in cycle length of 8 days or more.”
In the unvaccinated group, about 4% of people experienced a similar clinically significant change. In the vaccinated group overall, it was about 5%.
However, the study noted, the change in the group vaccinated twice within one cycle resolved itself “within two postvaccine cycles.”
Edelman said that while there wasn’t enough data to know for sure, these changes all appear temporary.
The study did not determine the cause of the variation in cycles, but noted, “Our results cannot be explained by generalized pandemic stress because our unvaccinated control group saw no changes over a similar time period.”
That result is borne out in a larger study of Natural Cycles users as well, Edelman said, that found pandemic stress alone did not have a meaningful impact on menstrual cycles.
So, what caused the change?
According to the study authors, “mRNA vaccines create a robust immune response or stressor, which could temporarily affect the hypothalamic-pituitary-ovarian axis if timed correctly.”
The authors believe this hypothesis is supported by the larger change found in those who received two doses in one cycle.
“Given the dosing schedule of the mRNA COVID-19 vaccines in the United States (21 days for Pfizer and 28 days for Moderna),” they wrote, “an individual receiving two doses in a single cycle would have received the first dose in the early follicular phase. Cycle length variability results from events leading to the recruitment and maturation of the dominant follicle during the follicular phase, processes known to be affected by stress.”
Edelman also noted that there wasn’t currently data that showed the impact of a COVID-19 infection on menstruation and fertility. However, the study authors wrote, “an acute severe illness with or without septicemia, such as COVID-19, could be catastrophic to hypothalamic-pituitary-ovarian axis function, sometimes permanently.”
Edelman said that there is still more to learn about how this vaccine and others impact menstrual cycles. But, she emphasized, for people concerned about how vaccination might impact their cycle, this study should be reassuring.
And for people still concerned about how the vaccine will affect fertility, beyond this possible minimal and temporary change in cycle length, Edelman said, “the data around fertility and pregnancy is super solid.”
“We haven’t seen anything that looks concerning whatsoever,” she said, adding that in fact, the counterfactual is true, which is that “actually getting the disease is horrible for pregnant women and individuals who can carry pregnancies.”
“We’re getting more and more data that this is incredibly safe, effective,” Edelman said, “and recommended for individuals that are trying for pregnancy and those that are pregnant.”
-- Lizzy Acker
503-221-8052, lacker@oregonian.com, @lizzzyacker