About 4 in 10 people with a constant menstrual cycle noticed more heavy bleeding after taking at least one dose of the Covid-19 vaccine. The proportion of participants who didn’t notice any changes in their period is about the same — and a smaller number noticed a lighter cycle after immunization, according to a study published in the journal Science Advances on Friday.
The survey is signed by researchers from American universities and was based on a questionnaire, available in 2021, with questions about the demographic profile, vaccination status and menstrual cycle of the participants.
After filtering, the researchers reached 39,000 responses. Participants could not have been diagnosed or suspected of Covid-19 and needed to have taken a dose of some vaccine against the disease for at least 14 days – the study considered immunizers from several manufacturers, such as Pfizer and Moderna.
With the data, the researchers observed that 42% of people with a constant menstrual cycle reported greater bleeding after vaccination against Covid. Meanwhile, 44% of participants didn’t notice a change and 14% didn’t notice a heavier flow — which could mean no change in bleeding or noticing lighter bleeding.
Some characteristics were associated with participants who reported greater bleeding. One of these factors was having had fever or fatigue after immunization. Other aspects were age – bleeding was more associated in older people – and racial – people of color and Latino had more reports of the change.
Reproduction-related issues also appeared as a factor associated with increased bleeding. Fertile women, those who were already pregnant or women who had given birth reported more cases of menstrual cycle changes.
The investigation also noted that some participants did not menstruate constantly, but described bleeding after taking a Covid-19 vaccine.
In this case, the researchers looked at three separate groups: post-menopausal women, those using birth control, and transgender participants using hormone treatment.
About half of postmenopausal women saw bleeding after vaccination – a similar rate was seen in those taking birth control. For transgender people, an average of one-third of them reported bleeding.
In most cases, changes in bleeding occurred up to seven days after vaccination. Some participants, however, reported the changes for up to two weeks.
Next steps
Even though the study found associations between greater post-vaccination bleeding and certain characteristics of the participants, it was not possible to establish a causality between the factors. According to the authors, the relationships seen in the research need to be understood in more depth.
One way is to investigate why more profuse bleeding was seen in those who reported fever and tiredness after vaccination. One hypothesis would be to understand the effect that an acute immune response of the organism after immunization would have on the menstrual cycle.
The authors also point out that changes in menstruation tend not to pose a danger to people’s health. However, developing research with the aim of understanding changes in menstruation is important, as there are still gaps.
One of these bottlenecks is that there are few studies on the menstrual cycle in trans and nonbinary people, say the authors. Bleeding is also understudied in women who no longer menstruate constantly. The authors, for example, did not identify other studies that considered women after menopause to understand the effects of vaccination on the flow of bleeding.
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