Here, we report the case of a man living with HIV who, having previously achieved stable viral suppression, experienced a transient period of non-suppressible viremia after receiving a dose of COVID-19 vaccine. The patient, a 65-year-old heterosexual male, was diagnosed with HIV infection in March 2020, during a diagnostic workup for chronic liver disease due to former alcohol abuse. In April 2021, the patient, by then suppressed for more than 6 months, with a negative clinical history and negative anti-N antibodies for SARS-CoV-2, received the first dose of mRNA-1273 vaccine (Moderna Biotech) in the context of the Italian vaccination campaign....