COVID-19 infection and vaccination can lengthen the menstrual cycle of a person, a study has found.

The human immune system and reproductive systems are interconnected and severe infections are known to affect the menstrual cycle. However, previous studies that evaluated the impact of COVID-19 infection on the menstrual cycle were inconsistent.

"Menstrual health is significantly understudied and underrepresented in research and medicine. This work is important and necessary to provide answers which can reassure individuals who are experiencing menstrual changes following a COVID-19 infection," said Dr. Alison Edelman, a co-author of the study.

For the study, researchers from the University of Montpellier and Oregon Health & Science University used data from a reproductive health app, called Clue, and evaluated the changes in the menstrual cycle of more than 6,000 participants from over 110 countries.

Participants were divided into three categories: a control group with no history of COVID-19 vaccination or infection, a group with participants who are vaccinated but without COVID-19, and a group that had COVID-19 infection (divided further into vaccinated and unvaccinated).

The unvaccinated participants had a 1.45-day increase in their menstrual cycle after experiencing a COVID-19 infection, while participants who received the COVID shot had a 1.14-day increase in the cycle.

A small proportion of participants reported a change in the menstrual cycle of more than eight days. However, the study showed the changes disappeared by the next cycle.

The changes in the menstrual cycle are believed to be due to the interaction of the virus with the immune system. The team has not done a causative analysis, although they plan to conduct further studies to explore the cause.

"People want to know when things change and why they change," Dr. Edelman said.

The findings provide reassurance to people that the changes in the menstrual cycle due to COVID-19 infection and vaccination are temporary, and they will soon get back to normal.

"For the majority of individuals, things get back to normal pretty quickly. And if they don't, they should be talking to their healthcare provider," researchers said.