Study: COVID-19 vaccination may cause temporary, slight change to menstrual cycle

COVID-19 vaccination may cause a temporary change to a person’s menstrual cycle, but it appears to be a “small change,” according to a study published on Thursday.

The study, published in the Obstetrics and Gynecology journal, examined close to 4,000 people — vaccinated and unvaccinated — and examined menstruation data that individuals entered through an app called Natural Cycles.

Researchers then examined the difference between menstruation data before and after individuals received their vaccine doses. For individuals who did not receive the vaccine, researchers looked at six consecutive menstruation cycles. 

The researchers found that in vaccinated individuals, bleeding was prolonged by a time that amounted to less than one day. However, they concluded that this slight change, while statistically significant, was not clinically significant.  

“After adjusting for confounders, we found that normally cycling individuals experienced small variations in cycle length regardless of vaccination status,” researchers said. 

“Statistically significant differences existed between vaccination status groups, but the change in cycle length was less than 1 day, which is below the reportable difference in the menstrual cycle tracking application and is not clinically significant,” the study added.

It is unclear why that slight, temporary change occurs, though the lead researcher of the study — Oregon Health and Science University professor Alison Edelman — told NPR that the reproductive system and immune system are tied.

“We haven’t seen anything that’s concerning regarding fertility or pregnancy in terms of vaccination,” Edelman told NPR. 

The doctor said that the changes to menstruation should ultimately be added to the list of possible minor side-effects for the COVID-19 vaccine such as pain in the arm, headache or fever. 

The National Institutes of Health announced last year that it was doling out grants to researchers to investigate the impacts of the COVID-19 vaccine on irregular or missed menstruation periods after some people noted they had noticed heavier bleeding than normal and irregular cycles.