Adin ME, Isufi E, Kulon M, et al. - The intramuscular administration of the COVID-19 mRNA vaccines is commonly followed by ipsilateral axillary nodal reactivity. This is more frequently observed after the second dose than after the first, and more commonly with the Moderna than the Pfizer vaccine.

• Moderna and Pfizer COVID-19 vaccine-related nodal reactivity on ¹⁸F-flurodeoxyglocose (FDG) positron emission tomographic (PET)/computed tomographic (CT) scans is described.

• Among 68 patients who received at least 1 dose of COVID-19 vaccine, overall 13% (n = 9) developed reactive ipsilateral axillary lymph nodes.

• Five percent (n = 2) developed ipsilateral axillary nodal reactivity after the first vaccine dose.

• Seven patients (26%) developed ipsilateral axillary nodal reactivity after the second vaccine dose; 4 patients after the second dose of the Moderna vaccine and 3 after the second dose of the Pfizer vaccine.

• This study had higher frequency of nodal reactivity relative to the Moderna trial after the second dose, which may be due to the high sensitivity of ¹⁸F-Flurodeoxyglocose–PET/CT for detection of reactivity in nonenlarged or enlarged lymph nodes.

• The COVID-19 vaccine should be administered in the axilla contralateral to the previously or potentially involved site in patients with cancer with a propensity for spread to ipsilateral axillary lymph nodes—breast cancer, melanoma, lymphomas.