Menstrual cycles are not as well known as pulse, breathing rate and body temperature, but they are still important signs of health.
In early 2021, when people started noticing that they had heavier than usual menstrual bleeding after they got vaccine against COVID-19, it was the only time.
Although these changes were short-lived, and most people's cycles returned to normal a month or two later, there was still a question about how many people had experienced changes in their periods after vaccinations.
Around 42% of people with regular menstrual cycles bled more heavily than usual after getting a vaccine, according to a study published in Science Advances.
Vaccine trials don't usually monitor participants for more than a week after the vaccine is given, and rarely ask about it.
There was no way for vaccine manufacturers to address the extent to which this observation was a coincidence or a potential side effect of the vaccine.
Lee joined forces with Kate Clancy to collect data about women's health at the University of Illinois.
The researchers designed a web-based survey to capture a wide range of responses from menstruating adults when multiple viral variants spread around the world.
Clancy told Science Alert that the response was overwhelming.
It's not new that vaccines could change menstrual cycles.
A study in 1913 found that the typhoid vaccine was linked to menstrualIrregularities, which include missed, late, early and heavy periods. Some people have had their menstrual cycles disrupted by the vaccine for human papillomaviruses.
Changes to menstrual bleeding are not uncommon or dangerous, but attention to these experiences is needed to build trust in medicine.
It's important for groups whose health concerns have been ignored by doctors.
The people who had not had COVID-19 were asked if they had noticed any changes in their period flow, cycle length, bleeding duration, or menstrual symptoms.
Lee and colleagues write that unexpected menstrual changes can cause concern, distress, or other negative responses.
Half of the people who responded to the online survey said they had more menstrual bleeding after getting a vaccine.
A majority of people reported no change to their period flow after vaccinations.
Less than half of people had light periods.
A lot of people who don't usually menstruate, but use long-acting contraceptives or gender-affirming hormones, also report breakthrough bleeding.
Lee, Clancy, and colleagues caution that the survey results may not generalize to wider populations.
Clancy told ScienceAlert that the point of this was never to perfect assess prevalence, but to listen to people and validate their concerns.
The study findings are in line with other studies of menstruati ng people in the UK and the Netherlands.
Clancy reported that survey respondents were angry that no one had studied the vaccine before they got it.
Researchers stress that the findings are not indicative of changes to fertility and suggest that further research is needed into possible causes of temporarily disrupted cycles.
Those who experienced heavier periods after getting vaccine were more likely to be older, Latino or Hispanic, and have pre-existing conditions.
Even the most regular of menstrual cycles can be affected by biological stressors. It is possible to disrupt or halt menstrual cycles without affecting fertility.
It's difficult to define how period length, timing and flow will change after you get a vaccine.
Lee and Clancy have a hunch about what's happening.
It's more likely that vaccines could be messing with the body's inflammatory pathways, rather than changing ovarian hormone pathways, because there was little difference between people who were taking hormones and those who weren't.
The only way to get rid of these effects is to collect the data from the beginning of a clinical trial.
Victoria Male wrote in the British Medical Journal that the effects of medical interventions on menstruation should not be an issue in future research.
The ideal setting for differentiating between menstrual changes caused by interventions from those that occur anyway is clinical trials, but participants are unlikely to report changes to periods unless specifically asked.
To see if the findings hold true in a larger sample and over time, Lee and Clancy are going to reanalyze the entire dataset from the first survey. They are looking at people's past experiences of healthcare.
Clancy told Science Alert that they hope to better understand how healthcare experiences affect vaccine sentiment, to improve trust and communication, and give patients the listening and caring treatment they deserve.
The study was published in a peer-reviewed journal.
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