Thousands of people have reported unusually heavy menstrual bleeding after receiving a COVID-19 vaccine, according to a new study in the journal Science Advances. The sample includes postmenopausal and transgender individuals who previously menstruated but currently wouldn’t expect a period at all – although the authors point out that there’s no reason to assume that these short-lived changes constitute a health risk.
Sudden recurrence of menstrual bleeding in those who no longer usually get a period is called breakthrough bleeding and can be distressing for numerous reasons. For instance, it can be a sign of uterine cancer, and those who experience this side-effect post-vaccination are therefore likely to become unnecessarily worried.
According to the researchers, studies that highlight the prevalence of this phenomenon are needed to alleviate such fears and combat misinformation about the effects of COVID-19 vaccines on fertility and general health. “Generally, changes to menstrual bleeding are not uncommon or dangerous, yet attention to these experiences is necessary to build trust in medicine,” they write.
The study presents the results of a social media survey, in which those over the age of 18 was invited to describe the impact of vaccination on their menstrual patterns. Conducted between April and October 2021, the survey attracted over 165,000 responses, of which 39,129 were analyzed.
Overall, 42 percent of participants reported a heavier-than-usual period after receiving a vaccine, with 44 percent describing no change in menstrual flow. Such findings are significant as clinical trials for vaccine safety typically don’t monitor menstruation and only consider symptoms that appear within a week of vaccination, although many survey respondents reported menstrual changes over seven days after a jab.
A more detailed breakdown of the data revealed that heavier bleeding was most common in those of Hispanic or Latinx descent, plus those who had previously been pregnant or been diagnosed with a reproductive condition such as endometriosis, menorrhagia, or polycystic ovary syndrome.
“Among respondents who typically do not menstruate, 71% of people on long-acting reversible contraceptives, 39% of people on gender-affirming hormones, and 66% of postmenopausal people reported breakthrough bleeding,” report the authors.
While the mechanism linking COVID-19 vaccines to unusual menstrual phenomena is unclear, the researchers believe that the cause is more likely to involve inflammatory pathways than ovarian hormonal disruption.
This assumption is backed up by the fact that those taking hormonal contraception were just as likely to experience heavy periods as spontaneous menstruators, thus indicating that the whole process is unlinked to hormone levels.
It’s important to note that this study did not include a control group, and is likely to have attracted responses predominantly from those who experienced troubling side-effects. As such, results may not be representative of the overall population and are likely to disproportionately highlight the experiences of those who noticed heavy or unexpected bleeding after vaccination.
Because of these limitations, the authors are keen to point out that they “cannot estimate prevalence or incidence based on our methodological approach of this emergent phenomenon, and the associations reported here cannot establish causality.”