Getting the flu vaccine may affect women's menstrual cycles, according to a new study by scientists.
Experts found that the vaccine, which is given to millions of women each year, slightly lengthens the length of the menstrual cycle. On average, the monthly period of vaccinated women was delayed by about a day.
Scientists suspect that this is due to the interaction that the vaccine causes between the immune system and female sex hormones.
These findings come after many reports from women who have complained of delayed or heavier than usual periods after receiving the vaccines.
Anti-vaccine campaigners have exploited this to spread fears that vaccines could affect women's fertility, although these claims have been refuted by scientists.
According to experts in this new study, it is very important to understand more about the interaction between vaccines and menstrual cycles to combat the growing hesitancy towards vaccination.
In this study, scientists from Oregon Health & Science University in the US analyzed data from over 1,500 women aged 18 to 45.
Of these, 791 had received only the flu vaccine, while 710 had received both the flu vaccine and the Covid-19 vaccine.
On average, women who had received the flu vaccine experienced a lengthening of their menstrual cycle by half a day.
About one in 20 women from both groups experienced greater changes, with their cycle being delayed by at least eight days after vaccination.
This change only occurred if the vaccine was given in the first half of the menstrual cycle — the follicular phase — when an egg begins to mature for ovulation. This phase begins on the first day of menstruation and lasts about 13–14 days.
As in previous studies, these changes were found to be temporary.
In the journal JAMA Network Open, American experts write that their findings could help doctors reassure patients worried about vaccine side effects.
They added that the link between vaccines and the menstrual cycle has been neglected in research and should be taken seriously.
It is not yet known exactly how vaccines cause these changes, but the fact that the impact only occurs in the first half of the cycle suggests that the immune response from the vaccine may interfere with hormone production, affecting the cycle.
The authors added that women who wish to avoid these disorders may choose to be vaccinated at a different stage of the cycle.
They also acknowledged that the study had some limitations, including a small and not very diverse number of participants.
The length of the menstrual cycle varies from one woman to another, but the average is every 28 days.
However, cycles lasting between 21 and 40 days are considered normal.
The NHS (National Health Service in Britain) advises women to consult their doctor if:
Their cycles suddenly become irregular and they are under 45 years old;
Periods occur more often than every 21 days or less often than every 35 days;
Menstruation lasts more than 7 days;
There is a difference of at least 20 days between the shortest and longest cycle.
Irregular cycles can also create difficulties for women trying to get pregnant, as it becomes difficult to accurately determine when ovulation occurs. /Express newspaper/
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