30,000 women have reported their periods being altered after getting the COVID-19 vaccine.
Research is needed to understand why this is happening, according to a paper in the British Medical Journal.
Periods can be heavier or delayed because of an immune response and poses no danger to one's body, say experts.
Since the rollout of COVID-19 vaccines, thousands of women in the UK have been saying that their periods have been disrupted, say experts.
More than 30,000 women said their menstrual cycle being somewhat altered after getting the COVID-19 vaccine, reported Sky News.
The UK's Yellow Card scheme, where people can voluntarily report their side effects to any medication - including vaccinations - has shown that many women have seen a disruption in their periods.
Dr. Victoria Male, a reproductive immunologist from Imperial College London, wrote in the British Medical Journal that while these changes are safe and short-lived, has stated that an investigation as to why this happens is crucial.
In the US, the National Institute of Health is investing $1.67 million into understanding how the COVID-19 vaccines impact periods.
Dr. Male states that periods can be heavier or delayed because of an immune response, and poses no danger to one's body.
"Robust research into this possible adverse reaction remains critical to the overall success of the vaccination program. One important lesson is that the effects of medical interventions on menstruation should not be an afterthought in future research," wrote Dr. Male.
Writing in The Telegraph, Caroline Criado-Perez, author of Invisible Women, said: "As with most clinical studies, the Covid-19 vaccine trials did not investigate menstrual cycle effects - in fact, in many trials women are wholesale excluded because of potential menstrual cycle effects."
There is no reason to be significantly concerned about menstrual changes and long-term impacts, writes Dr. Male, as the vast majority of those reporting the post-vaccine alterations state that normality ensues quickly.
Meanwhile, the data available shows that the COVID-19 vaccine has no adverse effects on fertility and pregnancy.
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