A European Medicines Agency (EMA) committee on Friday recommended adding heavy menstrual bleeding to the list of side effects of mRNA-based Covid vaccines made by Moderna and Pfizer/BioNTech, Reuters reported.

Reports of heavy periods – bleeding characterized by increased volume and/or duration that interferes with quality of life – have been seen in clinical trials, the EMA said.

The cases, which were mostly non-serious and transient, were reported after the first, second and booster doses of Pfizer-BioNTech’s Comirnaty and Moderna’s Spikevax, the committee added.

The regulator has now concluded that there is at least a “reasonable possibility” that heavy menstrual bleeding is causally linked to these vaccines.

A Moderna representative said he was aware of reports linking his vaccine to dysmenorrhea (menstrual pain).

“However, at this time we have not found a causal relationship with Moderna’s COVID-19 vaccine Spikevax.”

Pfizer and BioNTech could not be reached for comment.

The EMA also said there is no evidence that the menstrual disturbances experienced by some people affect reproduction and fertility.

mRNA vaccines do not cause complications during pregnancy in expectant mothers or their babies and are as effective in reducing the risk of hospitalization and death in pregnant women as in non-pregnant people, a review by the EMA Emergency Task Force has found.

Overall, the body of evidence continues to show that the benefits of these vaccines far outweigh the risks, the agency reiterated on Friday.

Irregular menstrual cycles can occur for a number of reasons, including underlying medical conditions as well as stress and fatigue. Health authorities emphasized that cases of infection with COVID-19 were also registered.