Objectives Reactive axillary lymph nodes (ALN) may occur post-COVID-19 vaccination. This may be confused with malignant nodal metastases on oncological imaging. We aimed to determine the reactive ALN incidence and duration on 18F-fluorodeoxyglucose positron emission tomography -- computed tomography (18F-FDG PET-CT), and its relationship with gender, age and vaccine type. Methods A retrospective study was performed. Two-hundred and four eligible patients had 18F-FDG PET-CT between 01 January 2021 and 31 March 2021, post-vaccination with Pfizer- BioNTech or Oxford-AstraZeneca vaccine. Image analysis was performed on dedicated workstations. SPSS was used for statistical analysis. Results Thirty-six per cent of patients had reactive ALN until 10 weeks post-vaccination; reducing in frequency and intensity with time. Women were more likely to have reactive ALN compared with men. The frequency and intensity were higher in patients aged <65 years compared with those aged ≥65 years. However, no difference was found between both vaccine types in our study cohort. Conclusions Physicians' awareness of COVID-19 vaccine-related reactive ALN on 18F-FDG PET-CT is important to avoid inappropriate upstaging of cancers.