A new study has found that it’s possible the COVID-19 vaccine can cause a small change to the length of a person’s menstrual cycle, but that this is temporary and not harmful.

Thanks to global collaboration and advancements in medical science, researchers were able to turn over several vaccines for the SARS-CoV-2 virus in record time. As with any tried and tested treatment, however, the life-saving prophylactic does carry some rare side effects, which got some people wondering if changes to their menstrual cycle could be linked to the jabs.

The new study, published in the journal Obstetrics & Gynecology, used data from the menstrual cycle app Natural Cycles to conduct the study. People logging their cycles on the app can consent to share de-identified data for use in research, and the total included (3,959) consisted of 2,403 vaccinated and 1,556 unvaccinated individuals.

The researchers focused on vaccinated and unvaccinated US residents aged 18 to 45 years old who reported having a menstrual cycle of average length (around 24 to 38 days). They then looked at six cycles for both groups, reviewing three consecutive cycles pre- and post-vaccine for the vaccinated group and six consecutive cycles for the unvaccinated.

Using this information, they were able to establish if and how menses length changed in the vaccinated group, and if these changes were significant when compared to unvaccinated people. Those in the vaccinated group largely received either the Pfizer or Moderna shot.

Overall, receiving a COVID-19 vaccine was associated with a lengthening of the menstrual cycle by less than a day compared to participants’ cycles pre-jab. There were no significant changes identified in the unvaccinated group compared to their baseline data. While the vaccinated group saw a small increase in the total length of the menstrual cycle, this wasn’t associated with an increase in the number of bleeding days, meaning their actual period stayed the same.

While the change existed, lead author Dr Alison Edelman of Oregon Health & Science University, Portland, said that the observed variations were within the range of normal variability. However, further research is needed to establish if the vaccine has any influence over other symptoms experienced during menstruation such as pain, emotional changes and heaviness of bleeding.

“It is reassuring that the study found only a small, temporary menstrual change in women,” said Dr Diana W. Bianchi, director of National Institute of Health’s Eunice Kennedy Shriver National Institute of Child Health and Human Development in a statement. “These results provide, for the first time, an opportunity to counsel women about what to expect from COVID-19 vaccination so they can plan accordingly.”