One in five women who took part in a survey said their menstrual cycles were affected after they received a covid jab, UK researchers have revealed.
The draft study suggested women who were on the contraceptive pill were almost 50 per cent less likely to notice any changes to their period.
But the odds increased by 49 per cent for women who smoked and by up to 70 per cent for those who had contacted covid or were suffering from long covid.
The research, which was based on self-reported evidence from almost 5,000 women who responded to a Facebook survey, was said by its authors to provide “reassuring data” because 80 per cent reported no changes.
But the researchers, from Oxford, Edinburgh and Bristol universities, said clinicians should consider counselling women about possible side-effects and advising them to seek medical advice if the symptoms were severe, lasted more than one cycle or involved bleeding between periods or after sex.
The UK’s drugs watchdog, the Medicines & Healthcare products Regulatory Agency (MHRA), and two other independent quangos are reviewing reports of suspected side-effects menstrual disorders after covid vaccination.
The MHRA says evidence to date “does not support a link” between the vaccine and changes to periods.
But a growing number of women have come forward to flag concerns. By November 10, the MHRA had been alerted through its “yellow card” scheme to 41,509 suspected reactions relating to a variety of menstrual disorders “including heavier than usual periods, delayed periods and unexpected vaginal bleeding”.
These reports relate to all three vaccines offered in the UK. The researchers said prior covid infection may heighten the body’s immune response to the vaccine, with a knock-on impact on menstruation, though this has yet to be proved.
The timing of the jab in relation to the phase of the menstrual cycle may also be a factor.
in a separate study of 1,273 women, Dr Victoria Male, a fertility expert at Imperial College London, was “unable to detect strong signals” to support the idea that covid vaccination was linked to menstrual changes.
But she said this may have been due to the smaller sample size used in her research. She said that investigating the possible link was important to maintain public trust in the vaccine programme.
She told the Standard it would be wrong to conclude from the Oxford research that 20 per cent of women were likely to experience changes after receiving a jab.
“The firmer finding is that people on hormonal contraceptives were less likely to report a disruption when surveyed in this way,” she said.
“If that’s true, it could support the idea that there’s a real biological link, mediated by sex hormones.”
Last month, pregnant women were urged to get the jab after unvaccinated expectant women, or those trying for a baby, were found to make up a fifth of covid patients in intensive care.