Vaccines against COVID-19 have demonstrated a remarkable efficacy in decreasing hospitalisations and deaths; however, clinical trials leading to vaccine approvals did not include immunocompromised individuals such as patients receiving antineoplastic therapies. Emerging data suggest that patients on active anti-cancer therapy may have a reduced immune response to COVID-19 vaccination compared to the general population and may be at greater risk of COVID-19 infection as measures to reduce transmission in the community are relaxed. We report preliminary data from the American University of Beirut Medical Center in Lebanon demonstrating relatively low seroconversion rates. Of 36 patients on active anti-cancer therapy who had received two doses of vaccine, 17% were negative for Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) anti-spike IgG. These results highlight the importance of maintaining strict precautionary measures against COVID-19 in patients on immunosuppressive treatment. There is an urgent need for active monitoring of immune response post-vaccination in prospective studies involving populations from diverse resource settings.