A link between changes in the menstrual cycle and COVID-19 vaccination is plausible, and should be investigated, according to an editorial published in the British Medical Journal on 16 September.

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In the editorial, Victoria Male, a reproductive specialist at Imperial College London, UK noted that periods or unexpected vaginal bleeding are not listed as common side effects of COVID-19 vaccination. Over 30,000 such reports had been made to the UK Medicines and Healthcare Products Regulatory Agency (MHRA) surveillance scheme for adverse drug reactions by September 2, she said.

The expert, however, noted most people find that their period returns to normal the following cycle and, importantly, there is no evidence that COVID-19 vaccination adversely affects fertility. "One important lesson is that the effects of medical interventions on menstruation should not be an afterthought in future research," Male said.

MHRA states that its surveillance data does not support a link between changes to menstrual periods and COVID-19 vaccines, since the number of reports is low in relation to both the number of people vaccinated and the prevalence of menstrual disorders generally. However, Male said the way in which data is collected makes firm conclusions difficult.

She argues in the editorial that approaches better equipped to compare rates of menstrual changes in vaccinated versus unvaccinated populations are needed. Reports of menstrual changes after COVID-19 vaccination have been made for both mRNA and adenovirus-vectored vaccines, she said.

This suggests that, if there is a connection, it is likely to be a result of the immune response to vaccination, rather than to a specific vaccine component, Male said. The editorial also noted that the menstrual cycle may be affected by the body's immune response to the virus itself, with one study showing menstrual disruption in around a quarter of women infected with SARS-CoV-2.

If a link between vaccination and menstrual changes is confirmed, this will allow individuals seeking vaccination to plan in advance for potentially altered cycles, she explained. She suggests that doctors should encourage their patients to report any changes to periods or unexpected vaginal bleeding after vaccination to the MHRA's scheme.