COVID vaccines may only affect the menstrual cycles of women on contraception, the first British study into the phenomena suggests.
Thousands of vaccinated women complained of irregular periods after getting the jab, and some postmenopausal women said they suffered abnormal vaginal bleeds.
But there has been no clear evidence of a link, even though similar problems can arise with vaccines for other illnesses.
Now, Imperial College London researchers believe they might have an answer after studying 1,200 women.
They found no definitive link between women receiving the jab and experiencing changes to the menstrual cycle.
But women taking contraception such as the Pill were more likely to have heavier or lighter periods than normal, compared to those not using them.
This was the case with all three vaccines used in Britain’s roll out — AstraZeneca, Pfizer and Moderna.
Results showed among those on contraception 42 percent said their periods were heavier than usual, and 19 per cent found they were lighter.
For comparison, for those not using the devices 32 per cent said they were heavier and 14 per cent warned they were lighter. The difference was statistically significant.
Results also showed endometriosis patients — a condition where tissue lining the womb grows in the ovaries — were significantly more likely to have an earlier period than normal compared to those not suffering from the condition.
And those with polycystic ovary syndrome (PCOS) — when the ovaries stop releasing eggs — were more likely to have a later period than others.
Dr Victoria Male, the immunologist who led the study, admitted it was ‘difficult’ to suggest a biological reason why women on contraception were more likely to experience period issues.
She said the results may in fact be down to women who are on the Pill monitoring their menstrual cycles more closely than others.
The Pill can also trigger period changes.
Dr Male told MailOnline: ‘A lot of people who take hormonal contraception at least partially want to control their periods.
‘It could be that they are more likely to report the change because the contraception normally makes their periods very regular — so it’s particularly unusual for them.
‘The other possibility [that vaccines are changing periods] is we do see a weak signal that some with PCOS might be more likely to have a late period.’
The study — published as a pre-print on medRxiv — asked for data from 1,273 women who kept a record of their menstrual cycle when they were vaccinated.
The group was aged between 29 and 39 years old.
Pfizer’s vaccine was administered to most participants (61 per cent), followed by the AstraZeneca jab (27 per cent) and Moderna shot (10 per cent).
Some 134 volunteers used hormonal contraception during the study, and 1,117 said they did not. Types of contraception deployed included the Pill, implants, a patch, and a vaginal ring.
Some 87 participants suffered from PCOS, and 60 had endometriosis.
Experts said the sample size was large enough to determine whether vaccines do affect the menstrual cycle.
But they added larger studies in the US would be able to observe rarer impacts that vaccines have on periods, which their study was too small to detect.
Experts have previously suggested the immune response triggered by vaccines could also be affecting periods because the two systems are heavily intertwined.
Some pregnant women and women looking to have a baby have been hesitant to come forward due to concerns about vaccines and fertility.
This is an excerpt from The Daily Mail.
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