Getting a flu jab can disrupt women’s menstrual cycles, scientists say.
Experts found getting the vaccine, administered to millions of women each year, did result in a small change to menstrual cycle length.
But the overall effect was minor, with an increase to cycle length of less than a day on average.
They also found this occurred regardless of if women only got the flu vaccine or also got a Covid jab at the same time.
During the initial rollout of the Covid vaccine tens of thousands of women complained about late or unusually heavy periods after getting jabbed.
At the time anti-vaxx campaigners leapt on the reports, using them to peddle fears that vaccines may affect fertility, though these were soon dismissed by scientists.
However, experts of the new study said it was critical to understand more about the phenomena and if it occurred in other jabs to help combat rising vaccine hesitancy.
In the latest research, scientists from Oregon Health & Science University in US analysed data from just over 1,500 women aged between 18 and 45.
Getting a flu jab can disrupt women’s menstrual cycles and this happens regardless of if you have a Covid vaccine, scientists say
Of these, 791 only had the flu vaccine and 710 had both it and the Covid jab.
On average women who had the flu jab saw their menstrual cycle length increase by 0.4 days.
This increased to an average of 0.49 days for women who got both jabs, a difference the authors said was negligible between the two groups.
About one in 20 women in both groups did experience a change in cycle of at least eight days after vaccination.
The authors also found that these changes to the menstrual cycle only occurred if women were vaccinated in the first half of their cycle.
Much like previous studies on the topic they found these changes were temporary.
Writing in the journal JAMA Network Open they said their findings should help medics reassure patients worried about vaccine side effects,
‘These findings may help clinicians confirm the utility of vaccination for patients with concerns about menstrual adverse effects of vaccination,’ they said.
They added the topic of how vaccines could influence the menstrual cycle had been neglected and it was important this was addressed.
‘This provides an important first data point about how influenza vaccination might affect menstrual cyclicity, a topic that has been largely ignored throughout the almost century-long history of influenza vaccines,’ they wrote.
‘Public concern about new vaccines creates mistrust about all vaccines.’
They said the exact mechanism of how vaccines caused changes in the cycle was unclear.
However, they added the fact it only seemed to impact women in the first half of their cycle suggested the immune response generated by a jab affected a woman’s production of hormones, leading to a disrupted cycle.
The authors said women wanting to avoid these delays to their cycle may wish to time their vaccinations accordingly.
They added that their study did have some limitations, notably their small and not very diverse sample size which could limit the findings.
The length of the menstrual cycle varies between individual women, but the average is every 28 days.
However, cycles as short as 21 days or as long as 40 are not considered unusual.
The NHS advises women to contact their GP if their periods are irregular in certain situations.
These include: if their periods suddenly become irregular and they are under 45-years-of-age, their periods come more often than every 21 days and less often than every 35 days, their period lasts longer than seven days, and there is a difference of at least 20 days between the shortest and longest menstrual cycle.
Irregular periods can also lead to problems when women are trying to get pregnant as it becomes difficult to accurately track ovulation, the time when they are most likely to conceive.
