More than 30,000 women in the UK have reported disruption to their periods after having the coronavirus jab.
But changes are 'short-lived', according to an article in the leading medical journal, The BMJ.
The opinion piece written by Dr Victoria Male, lecturer in reproductive immunology, said most women return to normal after a single cycle and there was no evidence that vaccination affected fertility.
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But Dr Male called for more research to ensure the success of the vaccination programme.
She said that vaccine hesitancy among women was largely driven by “false claims that vaccination could harm their chances of future pregnancy”.
“Failing to thoroughly investigate reports of menstrual changes after vaccination is likely to fuel these fears,” she wrote.
“If a link between vaccination and menstrual changes is confirmed, this information will allow people to plan for potentially altered cycles.
“Clear and trusted information is particularly important for those who rely on being able to predict their menstrual cycles to either achieve or avoid pregnancy.”
Common side effects from the Covid jab listed by the Medicines and Healthcare products Regulatory Agency (MHRA) include sore arm, fever, fatigue and muscle aches and pain.
But changes to periods are not listed.
“Primary care clinicians and those working in reproductive health are increasingly approached by people who have experienced these events shortly after vaccination,” she added.
“More than 30, 000 reports of these events had been made to MHRA’s yellow card surveillance scheme.”
Dr Male continued: “MHRA states that evaluation of yellow card reports does not support a link between changes to menstrual periods and Covid-19 vaccines since the number of reports is low relative to both the number of people vaccinated and the prevalence of menstrual disorders generally.
“However, the way in which yellow card data are collected makes firm conclusions difficult.”
She added: “Most people who report a change to their period after vaccination find that it returns to normal the following cycle and, importantly, there is no evidence that Covid-19 vaccination adversely affects fertility.”
The lecturer at Imperial College London said menstrual changes had been reported among women who had received both mRNA vaccines – such as the Pfizer and Moderna jabs – and adenovirus vectored Covid-19 vaccines, such as AstraZeneca.
She added that if there was a connection between vaccines and disrupted periods “it is likely to be a result of the immune response to vaccination rather than a specific vaccine component”.
Dr Male concluded: “Although reported changes to the menstrual cycle after vaccination are short-lived, robust research into this possible adverse reaction remains critical to the overall success of the vaccination programme.”
Research would also help understand any potential mechanism behind the potential link, she added.
Dr Jo Mountfield, vice president of the Royal College of Obstetricians and Gynaecologists, said: “We understand that any changes to periods following a Covid-19 vaccine can be concerning.
“We want to reassure women that any changes generally revert back to normal after one or two cycles.
“We would encourage anyone who experiences heavy bleeding that is unusual for them, especially after the menopause, to speak to a healthcare professional.
“There is no evidence to suggest that these temporary changes will have any impact on a person’s future fertility or their ability to have children.
“It is important to get vaccinated as the best protection against coronavirus. This is especially important if you are planning a pregnancy, as we know unvaccinated pregnant women are more at risk of becoming seriously ill from Covid-19.
“We support calls for more research to understand why women may be experiencing changes to their menstrual cycle after having the vaccine.”
