Doctors are noticing an uptick of abnormal mammograms as more people get their COVID-19 vaccine. The vaccine is causing lymph nodes under the arm to swell as the body starts building protection against the virus. This alarming but harmless pattern is a common side effect of most vaccines, say experts.

“We have been seeing swollen glands for a couple of weeks now,” said Dr. Holly Marshall, a diagnostic radiology specialist in Cleveland, Ohio, according to the Miami Herald. “It’s actually a normal response that the body has to the vaccine. It means that the body is making antibodies to fight the COVID-19 infection.”

The swollen lymph nodes are called technically called axillary adenopathy, and on screening mammograms they could be mistaken for breast cancer, according to News Channel 8, a Fox news affiliate in Tampa.

“We also see swollen lymph nodes in cancer patients, so that’s the concern there,” Marshall said. “So, we are asking everyone how is having a mammogram if they had the COVID-19 vaccine, what dose, when, and what side.”

A Centers for Disease Control and Prevention report found over 11% of those who received the Moderna COVID-19 vaccine experienced swollen lymph nodes after the first dose and 16% following the second dose. The Pfizer-BioNTech vaccine also caused lymph node tenderness, according to the Herald.

Patients usually encounter swollen lymph nodes 2-4 days after the vaccine, and the swelling should subside within two to four weeks. If the symptom persists, patients should be evaluated further, Marshall said.