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Objective:
Differences between the natural course and incidence of reactive axillary lymph nodes (RAL) secondary to the various COVID-19 vaccines remain poorly understood. Reported incidences of RAL vary from 10-100%. In this multi-national PET-CT study comprising multiple vaccine types (Pfizer-BioNTech/Comirnaty, Moderna, CoronaVac and Janssen vaccines), we evaluated the incidence and natural course of RAL in a large cohort of oncological patients, utilizing previously validated RAL criteria.

Materials and Methods:
Data collection methodology was similar to a recent pilot study. Clinical parameters collected included age, sex, SARS-Cov-2 infection status, indication for PET/CT scan, time between the vaccination and PET/CT scan, vaccine parameters (commercial brand, first vs second dose, administration site) and blood parameters (absolute neutrophile [ANC] and platelet counts). PET parameters included SUVmax and SUV mean values for mediastinal blood pool, liver, axillary lymph nodes, extremity injection site, activity, and presence of avid lymph nodes in the contralateral axilla and rest of the body. The lymph nodes with FDG uptake (SUVmax) below or at the blood pool level were classified as nonreactive, and those with FDG uptake above the blood pool level were classified as reactive. Using the Deauville scoring system, we aimed to create a universally replicable parameter, as absolute SUV may differ between individuals and institutions. Deauville (D) scores for lymph nodes were calculated (D1-2: nonreactive, D3-5: reactive).

Results:
Pfizer-BioNTech/Comirnaty and Moderna vaccines revealed similar RAL incidences for the first 20 days after the second dose of vaccine administration (44 % for the first 10 days for both groups, 26% vs 20% for 10 - 20 days), but Moderna recipients revealed significantly higher incidences of RAL after 20 days (4% vs 15% for Pfizer-BioNTech/Comirnaty and Moderna, respectively). No RAL was found in patients who received either a single dose of J&J vaccine or two doses of Sinovac. Except for age, other clinical parameters were not predictive of RAL. There was no significant difference for additional factors such as absolute neutrophil count, platelet count, sex or incidence of RAL in solid versus hematological malignancies.

Conclusion:
There is lower incidence of vaccine-related RAL in attenuated whole-virus vaccines, which is in line with their lower immunogenicity compared to mRNA vaccines. Stronger immunogenicity is presumably responsible for the increased likelihood of RAL in young patients.