Background: HIV superinfection is the re-infection of an HIV⁺ individual with a new HIV quasi-species. We hypothesized that superinfection is most likely to occur in persons with ongoing risky HIV transmission behaviours.

Methods: We conducted a retrospective cohort study of HIV patients followed for at least 2 yrs at the Ottawa Hospital Immunodeficiency Clinic. Low risk individuals (LORISK), abstinent from IDU and sex or in a monogamous relationship with an HIV-negative partner were compared with those at high risk (HIRISK) who continued IDU or unsafe sex. Amplifiable follow-up sera were compared to baseline sera. Superinfection was determined by phylogenetic analysis for the development of marked genetic diversity in envelope gene.

Results: Preliminary results are completed for 18 cases thus far; 88.9% (16/18) were B subtypes. There was no significant difference in gender, age, or antiretroviral therapy use between LORISK and HIRISK individuals. Superinfection was not associated with gender (p = 0.6), age (p = 0.3), antiretroviral therapy use (p = 1.0) or time interval between samples (p = 0.3). Superinfection developed in 46% of the HIRISK group (6/13), yet 0% in the LORISK group (0/5).

Conclusion: Initial infection with an HIV strain does not necessarily provide protection against superinfection with a second HIV strain. Strikingly, we found superinfection only in those who continued risky HIV transmission behaviours. These preliminary results are limited by small sample size to date. These highly provocative findings, if confirmed, have significant implications for vaccine development and suggest the need for superinfection prevention strategies in HIV-infected persons.