Axillary Adenopathy After COVID-19 Vaccination

COVID-19 vaccination in the upper arm can cause transient ipsilateral axillary lymphadenopathy, which complicates the interpretation of mammography and other imaging procedures that visualize the axilla. In this study from Japan, researchers compared findings on axillary magnetic resonance imaging (MRI) in 433 people in mid-2020 (before vaccination was available) and in mid-2021 (after participants had received 2 doses of mRNA vaccine). These yearly whole-body MRIs were part of a “comprehensive health screening program” unrelated to COVID-19.

Axillary adenopathy was defined as at least one postvaccination lymph node — with short-axis diameter ≥5 mm — that was larger than the corresponding prevaccination node. Findings were as follows:

• The prevalence of adenopathy was substantial initially and decreased over time — 65% when vaccination was within the previous 2 weeks, 40% when vaccination was 3 to 4 weeks previously, and uncommon after 8 weeks.
• The prevalence was higher with the Moderna vaccine than with the Pfizer vaccine (40% vs. 19%), and higher in women than men (29% vs. 17%).
• Most people with adenopathy had only 1 or 2 enlarged nodes.
• Only four people had nodes ≥10 mm in short-axis diameter.