Don't get breast exam soon after your COVID-19 vaccine, experts say

• A side effect of the COVID-19 vaccine can be swollen lymph nodes in the armpit near where you got the shot.
• It's not a cause for concern, but could cause false alarm in a breast exam.

Breast radiologists have noticed swollen or tender lymph nodes in the armpit following COVID-19 vaccinations, according to a recent study.

It's a harmless side effect that fades within days, but if swollen lymph nodes are detected during a breast MRI scan, it can cause concern, leading to more tests, undue anxiety, and even an unnecessary biopsy.

In the study, researchers at the Hospital of the University of Pennsylvania said they are already seeing harmless swollen lymph nodes in women who had recently gotten vaccinated shortly before their scheduled MRI. The swelling was always on the side of the vaccination site.

Dr. Laura Dean, a breast radiologist at the Cleveland Clinic who was not involved in the study, told Insider she has seen the same in mammograms at her own clinic.

It's hardly unexpected: this was a side effect identified in the Moderna clinical trial. They found that approximately 10% of vaccine recipients had swollen lymph nodes after their first dose, compared to 4.8% in the placebo group. Some 14% of participants experienced the same after the second dose.

Experts say the swelling, which usually fades two to four days after vaccination, shouldn't be a concern - it's a sign the vaccine is stimulating the immune system. That said, they recommend waiting to get a breast exam.

Swelling is a sign that the vaccine is working

Dean said it's well known that vaccines for the flu and smallpox have the potential to enlarge lymph nodes.

"Your immune system is mounting that response and it's doing exactly what it's supposed to do," Dean told Insider.

The same is being seen in people who've received the Pfizer and Moderna vaccines. So far, the rate is slightly higher than in other shots.

This side effect prompted Dean's office to change patient intake forms. Now they're asking questions about whether a patient has been recently vaccinated, which manufacturer it was from, and which arm was injected, to account for benign swelling.

Wait 10 to 14 days for your swollen lymph nodes to go down

Dean said when lymph nodes are enlarged on one side of the body, it's usually harmless. But in rare cases, it can signal breast or lymphatic cancer, which would ordinarily warrant a biopsy.

For people who have received the vaccine but need a timely a breast exam, Dean suggested scheduling an ultrasound to get a closer look, and then following up four to twelve weeks after to see whether any detected swelling had faded.

The Society of Breast Imaging recommends scheduling a screening exam before vaccination, or four to six weeks after the second dose.

Since swollen lymph nodes are so common after vaccination, Dean said, most women will not need a biopsy.

"It's always a risk and balance of being careful enough that we're not missing something such as cancer. But then, of course, not putting patients through the undue stress and slight discomfort of having a biopsy," Dean told Insider.