FORT MYERS, Fla. — As people around the world get vaccinated against coronavirus, some have said period changes were an unexpected side effect. After Katherine Lee got her COVID-19 vaccine, she talked with a colleague about side effects. They expected side effects like injection site pain and a slight fever, but they both noticed a strange symptom: their menstrual cycles changed. Lee talked with other people who got periods, and she heard that they also experienced flows that came earlier, felt heavier, or just seemed abnormal. “The menstrual cycle is a really flexible and dynamic process and it responds to a lot of different things in life like stress, physical or mental or immune changes,” Lee, the post-doctoral scholar in the public health sciences division at the University of Washington School of Medicine in St. Louis, said. “The menstrual cycle is supposed to respond and adapt.” Lee reached out to her grad school professor, Kathryn Clancy, head of the Clancy Lab at the University of Illinois, which focuses on women’s health research. She mentioned the period changes and Clancy was interested. Then she received her first dose. “A little after a week after this first Moderna dose and I had never had a period that was so heavy — not even in my 20s when I was having a really heavy cycle,” Clancy said. Clancy shared her experience on Twitter and people kept the conversation going with stories of their own period changes. “A lot of people had noticed something but hadn’t heard anything about (menstrual changes) being a side effect,” Lee said. “So many things could impact people’s menstrual experiences. So, we just thought if this is a side effect of … this type of vaccine it would be good for people to be prepared.” Both researchers said that they are pro-vaccine and they’re conducting the research to understand the full range of potential side effects. “We need to do more work noticing when there are different effects for different people, really, so that we can do a better job of (preparing for) these side effects,” Clancy explained. “If people know, for instance, this is going to make you bleed more they’re going to have more pads with them.” CLICK HERE to go to NBC2’s Vax Facts page with all the information you need to secure a vaccine appointment. Past vaccine studies & menstruation It’s not known if past studies on other vaccines looked at whether they influence period changes, experts said. Clancy said that it wasn’t until the 1990s when the National Institutes of Health required women to be included in studies. “We make a lot of assumptions about vaccines and side effects based off of data that doesn’t actually represent all bodies,” she said. “There are biological and cultural effects to all sorts of different phenomena, and we really need to do due diligence to study these.” While researchers do not understand how the vaccines might influence menstruation, they do have some understanding of how having COVID-19 impacts menstruation. “There are some studies that show that how the COVID virus actually enters the human cells and these receptors are found in part the GI system, kidneys, possibly the uterus, possibly the placenta,” Dr. Anar Yukhayev, an OB-GYN at Long Island Jewish Medical Center, said Yukhayev wonders if the inflammation that plays a role in the virus contributes to the changes. “Perhaps not the virus itself, but perhaps it is the antibodies and the inflammation reaction that that that it’s creating throughout the body,” he said. Yukhayev said people should talk to their doctors if they notice period changes and are worried. Some people who notice a change might want to take a pregnancy test. Others might be experiencing a change in menstruation for reasons unrelated to the vaccine, such as fibroids, endometriosis or polycystic ovarian syndrome (PCOS), especially if it is longer lasting.