Acute Hepatitis a in Immunized MSM Patients With Previous Positive Serology: Report of Two Cases

Hepatitis A (HAV) is generally self-limited but may progress severely in adults. The presence of anti-HAV IgG antibodies, either due to vaccination or prior infection, is considered a marker of long-lasting immunity. However, rare cases of acute hepatitis A have been reported in previously immunized individuals, particularly among people living with HIV (PLHIV). In immunocompetent individuals, documented cases of reinfection are lacking, making such occurrences exceptional. This study describes two cases of men who have sex with men (MSM) diagnosed with acute hepatitis A despite prior serology compatible with immunity, discussing possible causes such as declining antibody titers over time, vaccine failure, false-positive results, and potential associations with sexual practices or antiretroviral therapy (ART). A retrospective review of medical records was conducted for patients hospitalized in a tertiary hospital in São Paulo in 2024 with a diagnosis of acute hepatitis A. Cases with prior positive anti-HAV IgG serology were included. A literature review was also performed in PubMed, Science Direct, and SciELO using descriptors related to immunity, hepatitis A, vaccination, and reinfection. After screening 271 studies, 17 articles were analyzed. Case 1: male, MSM, 45 years old, HIV-negative, on PrEP, developed acute hepatitis A with positive anti-HAV IgM despite having positive anti-HAV IgG three months earlier. Case 2: male, MSM, 46 years old, living with controlled HIV, developed acute hepatitis A; he had positive anti-HAV IgG in 2017, which became negative in 2021 and 2023, and presented positive anti-HAV IgM during hospitalization. Both patients had favorable clinical outcomes with supportive treatment. While cases of acute hepatitis A in previously immunized PLHIV have been reported, similar cases in immunocompetent individuals are rare. Occurrence in a patient without known immunosuppression raises hypotheses of waning protective immunity, influence of specific sexual practices, or possible indirect effects of PrEP/ART use. The reported cases suggest the possibility of HAV reinfection even in individuals considered previously immune, including those without known immunosuppression. These findings highlight the importance of serological surveillance in vulnerable groups and the need for studies clarifying the duration of vaccine-induced immunity and the impact of sexual behaviors or PrEP/ART use.