Harmless vaccine side effect could mimic cancer in mammograms


One consequence of an active immune response can be an enlarged lymph node. And, because coronavirus vaccines activate the immune system, some people have swollen nodes in the days following a vaccine.

These are harmless if uncomfortable side effects – but they can be misleading when scanned by a radiologist, including during a mammogram. Here’s why.

What is this side effect?

After vaccination, a swollen lymph node may appear as a lump in the armpit. These glands are hotbeds of immune activity, filtering pathogens and storing germ-fighting cells. If you’ve had a sore throat or a cold, there’s a chance you’ve felt a swollen node in your neck. The post-vaccine node may be palpable, too.

This side effect appears to be more common after the Moderna and Pfizer shots than after the Johnson & Johnson vaccine. In Moderna’s trials, about one in six recipients reported swelling or tenderness in armpit lymph nodes, often on the same side as the shot, after a second dose.

Any swelling should resolve within days, and if it does, it isn’t a cause for concern. However, should it persist for multiple weeks, then it’s not a bad idea to notify your doctor.

Can a vaccine affect a mammogram?

Because armpits are captured in mammograms, swollen nodes may appear as alarming white spots on the images. It’s not possible for radiologists to discern the origin of the swelling – raising the possibility that they could be mistaken for metastasized cancer, which is what happened in five case reports published last month in the journal Radiology.


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