Thousands of women have reported changes in the periods after getting jabbed, such as their cycle running late and heavier or unexpected flows.
These changes are not listed as common side effects for vaccination but more than 30,000 reports had been made by 2 September to the UK Medicines and Healthcare Products Regulatory Agency (MHRA).
However, most people reported that the changes were temporary and that their period returns to normal the following cycle.
The MHRA states that its data does not support a link between changes to periods and Covid vaccines, as the number of reports is low compared with the number of people vaccinated and the prevalence of menstrual disorders overall.
Dr Victoria Male, a reproductive specialist at Imperial College London, said that the way in which data is collected renders making firm conclusions difficult, but that a link between menstrual changes after Covid vaccination is plausible and that it should be investigated.
In the British Medical Journal (BMJ), she wrote that one of the important “lessons” that emerged from the possible link between the vaccine and period changes is that the “effects of medical interventions on menstruation should not be an afterthought in future research.”
Reports of menstrual changes after Covid vaccination have been made for both mRNA and adenovirus-vectored vaccines, Dr Male 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 component of the vaccine.
Also, there is a study showing menstrual disruption in around a quarter of women infected with the actual Covid virus.
If a link between vaccination and menstrual changes is confirmed, this will allow individuals seeking to get vaccinated to plan in advance for potentially altered cycles, Dr Male wrote in the BMJ editorial.