Changes to periods after a Coivd-19 jab are “small” and “quickly reverse”, an expert has said.
More than 36,000 women reported changes to their periods to the Medicines and Healthcare products Regulatory Agency (MHRA) after having a vaccine in the UK.
But Dr Victoria Male, lecturer in reproductive immunology at Imperial College London, said that the latest evidence on the changes to a woman’s cycle are “limited but reassuring”.
In an editorial published in The BMJ, Dr Male pointed out that cycles can vary naturally.
She highlighted a study in the US, which found that the first dose of vaccine was not linked to changes to the timing of a woman’s next period.
But after a second vaccine there was an average delay of 0.45 days.
The women most affected were those who received both doses in the same menstrual cycle, who experienced an average delay of 2.32 days.
However it is unlikely that many people in the UK will have had two doses in the same cycle as British vaccination policy encourages longer gaps between vaccines.
The study also found that cycle lengths returned to normal after a couple of months.
Meanwhile a separate study from Norway asked women about changes to their cycles both before and after vaccine doses.
Some 38% of women reported changes before they had even been vaccinated.
But the study identified heavier than normal bleeding as the change most associated with vaccination.
Dr Male wrote: “The findings from both these studies are reassuring: changes to the menstrual cycle do occur following vaccination, but they are small compared with natural variation and quickly reverse.”
She added: “The MHRA says that current evidence does not support a link between changes to menstrual periods and Covid vaccination in the UK, and it continues to advise that anyone noticing a change to their periods that persists over several of cycles, or who has any new vaginal bleeding after the menopause, be treated according to the usual clinical pathways.”