- The UK's medicines regulatory agency got 27,199 reports of menstrual-cycle changes after COVID-19 shots.
- An OB-GYN said pandemic-related stress could have caused some of these changes.
- People trying to get pregnant should get vaccinated or risk getting "seriously ill," another doctor said.
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The UK's medicines regulatory agency has received more than 27,000 reports of period changes after COVID-19 vaccines in the past nine months, but leading doctors still urge people who are trying to get pregnant to get vaccinated.
Dr. Jo Mountfield, vice president of the Royal College of Obstetricians and Gynecologists, said in a statement on Thursday that any period changes after COVID-19 vaccines generally revert back to normal after one or two cycles. Plus, she added, unvaccinated pregnant women were more at risk of "becoming seriously ill from COVID-19."
"There is no evidence to suggest that these temporary changes will have any impact on a person's future fertility or their ability to have children," Mountfield said.
Dr. Jackie Maybin, a gynecologist at the University of Edinburgh, said in a statement that it was "very difficult to know if these changes are a direct effect of the vaccine itself or are due to wider effects of the pandemic."
"In times of stress, the female system is designed to temporarily downregulate to prevent against pregnancy and conserve energy. This brain-level effect may explain some of the changes in menstruation observed during the pandemic, with COVID-19 or with vaccination," Maybin said.
She added that any effects related to the vaccine are likely to be short lived and much less severe than a coronavirus infection.
A study from September 2020 of menstruating women showed that around a quarter of those who got COVID-19 experienced period problems.
Mountfield cautioned that anyone who experiences unusual heavy bleeding, especially after menopause, should speak to a healthcare professional.
27,000 reports of period changes after the COVID-19 vaccine
Data from the Medicines and Healthcare products Regulatory Agency (MHRA) released on Thursday showed that it received 27,199 reports of period changes after a COVID-19 vaccine - including heavier-than-usual periods, delayed periods, and unexpected vaginal bleeding - between December 9, 2020, and September 8 of this year.
Most changes were short-lived, it said. Providers gave 47.8 million COVID-19 vaccine doses to women during that time period, the MHRA said.
Maybin said the immune response from COVID-19 vaccines may cause inflammation that temporarily affects the ovaries, altering hormone production over one or two cycles and leading to irregular or heavier menstrual bleeding.
The immune response could also temporarily affect how the womb lining sheds, causing a heavier period, she added.
"These effects could lead to temporary changes in menstrual symptoms that should spontaneously resolve," Maybin said.
Other factors that affect periods include contraception, herbal supplements such as soya, sexually transmitted infections, and other medical conditions.
More research is needed
The MHRA said that its data, which is all self-reported by patients, couldn't be used to "derive side-effect rates" because many factors could influence the reports. Several groups are still reviewing the data, the agency said: the MHRA itself, independent experts from the Commission on Human Medicines' COVID-19 Vaccines Benefit Risk Expert Working Group, and the Medicines for Women's Health Expert Advisory Group.
Dr. Victoria Male, a reproductive immunologist at Imperial College London, wrote in an opinion piece published in the British Medical Journal on Thursday that clinical trials should investigate COVID-19 vaccines' impact on periods.
"Clear and trusted information is particularly important for those who rely on being able to predict their menstrual cycles to either achieve or avoid pregnancy," she said.
Not knowing whether there is a link could fuel vaccine hesitancy, she added.