Vaccines against COVID-19 may cause changes in menstruation in some women, but these changes are brief and minor compared to the natural variation exhibited by normal period cycles.
This is confirmed by two studies published this Wednesday by the British Medical Journal (BMJ) and whose conclusions are “reassuring”, as Victoria Male, a reproduction specialist at Imperial College London (United Kingdom), points out in a statement.
The expert, who has analyzed both works in an editorial for the BMJ, acknowledges that the preparations against COVID-19 does affect the menstrual cycle, although its effects are not significant or long-lasting.
The first study analyzed data from 3,959 women in the United States who recorded at least six consecutive menstrual cycles on a tracking app. Of that total, 2,403 were vaccinated and the rest joined the control group.
After accounting for other factors, the researchers found no change in menses after the first dose, while the second injection was associated with a 0.45-day delay.
The most obvious effects were recorded in 358 women who received the full vaccination schedule during the same menstrual cycle, which caused their next period to be 2.32 days late.
Within this group, the 11% experienced a change in cycle length of more than eight days, compared to 4% recorded in the unvaccinated group.
Victoria Male stresses that all groups returned to normal within two menstrual cycles after vaccination.
The second study examined the responses of five thousand 688 women in Norway who were asked if they had experienced specific changes in their periods (such as unexpected bleeding or pain that was more severe than normal) in the cycles before and after each dose of the vaccine.
The 38% of participants reported at least one change in cycles prior to vaccination, although that number rose to 39% after the first dose and 41% after the second.
The women surveyed revealed that, After vaccination, the most common change was heavier than normal periods.
“These results are reassuring. Changes in the menstrual cycle do occur after vaccination, but these are minor compared to natural variations and are quickly reversed,” observes Male.
However, it specifies that in some countries, such as the United Kingdom, the vaccination program does not allow the administration of the complete schedule during the same menstrual cycle, so the conclusions drawn in the US and Norway would not necessarily be repeated in other jurisdictions.