Getting a mammogram soon after getting the COVID-19 vaccine? Be warned: The vaccine might cause your lymph nodes to swell, which may be confused for a sign of breast cancer.

“Getting the COVID-19 vaccine can cause temporary lymph node enlargement and result in a ‘false positive’ mammogram,” Lisa Ann Mullen, MD, explained in an article published by Johns Hopkins Medicine on the topic. The enlargement may cause “your mammogram to appear abnormal even when you are OK and there is no indication of cancer.”

While the reaction is “nothing to worry about” and is a common effect of all vaccinations, it has caused a rise in swollen armpit lymph nodes on multiple types of breast imaging, leading many women to have additional testing, and sometimes even biopsies, as well as “unnecessary stress and anxiety,” Penn Medicine wrote in a post.

To avoid unnecessary further testing or distress, the Centers for Disease Control and Prevention advises that people ask their doctors how long they should wait before getting a mammogram after being vaccinated. “Some experts recommend getting your mammogram before being vaccinated or waiting four to six weeks after getting your vaccine,” the CDC adds.

The reason for the vaccine-induced swelling is quite natural.

“The whole point of the vaccine is to get your immune system to mount a response to whatever the vaccine agent is,” diagnostic radiologist Laura Dean, MD, told the nonprofit academic medical center the Cleveland Clinic for an article about the possibly alarming side effect. Lymph nodes, which are part of the body’s immune system, are thus known to swell in response to any and all vaccines, although increased reports suggest the coronavirus shot is causing “more robust swelling in lymph nodes” than others. The symptom appears to subside in “a few days to a few weeks,” Dean noted. 

Getting the COVID-19 vaccine could lead to a false positive for breast cancer.Getty ImagesWhen not induced by a vaccination, swollen underarm lymph nodes can be a sign of breast cancer. 

“If breast cancer moves outside of the breasts, it tends to go to those lymph nodes under the arms because that’s the natural drainage pattern of the lymph fluid inside the breast tissue,” Dean said. “It’s a very integrated system, so it’s one of the areas we closely scrutinize.”

The situation is a highly predictable one, yet the failure of authorities and medical professionals to inform the public about it has many people spiraling — convinced they have cancer following what they expected to be routine checkups. 

Jezebel reporter Shannon Melero’s mother was left “mentally planning her own funeral” after she was informed something that was “most likely benign” had been “found” following a recent mammogram. The scare, however, turned out to be a false flag triggered by her having received her second COVID-19 vaccine dose days earlier, Melero reported.

The vaccine’s impact on mammograms isn’t the only topic that the medical industry has barely warned the public about. Many are also reporting that the vaccine caused them to have heavier and, in some cases, unanticipated periods.

“It wasn’t a symptom that was on the list,” Washington University research fellow Katharine Lee, Ph.D., told the Verge of having an unusual menstrual cycle soon after receiving the vaccine. In response, she co-launched a formal study on COVID-19 vaccines’ impact on the menstrual cycle. 

“These are just not things some people think about,” she said. “It’s not part of their daily lived experience. I think a lot of it is related to that history, and the bias around who gets to decide what’s important to consider as a side effect.”