Introduction: Estimating Human Immunodeficiency Virus type 1 (HIV-1) infection remains one of the great challenges of the epidemiologic surveillance of HIV-1 epidemics. WHO has recently approved a laboratory based strategy that enables the identification of recent infections.
Objectives: To identify the number of recent (incident) infections and estimate the seroincidence of HIV-1, using for the first time in Portugal, a new methodology based on the Avidity Index of HIV-1 antibodies.
Methods: Cross-sectional study of a HIV-1 positive group selected within drug users admitted in a Low Threshold Methadone Program in Lisbon during a one-year period. Avidity Index is calculated by testing a sample from each participant on the automated AxSYM HIV 1/2gO assay (Abbott) following a specific protocol. HIV-1 infections are classified as recent or established according to the Avidity Index value.
Results: The Low Threshold Methadone Program admitted 714 drug users and 175 were HIV-1 infected at admission (proportion of 24.5%; 95% confidence interval (CI): 21.3-27.7). Twenty recent infections were identified and the seroincidence of HIV-1 estimated as 3.58% (IC 95%, 2.0-5.1) (six-month value), which corresponds to an annual projection of 7.16% (IC 95%, 5.0-9.3). Comparative analysis between groups identified independent associations between incident HIV-1 infections and race/ethnicity (p=0.047), educational level (p=0.006) and presence of HBsAg (p=0.028). In the present study no association was found between incident HIV-1 infections and ever injected or syringe sharing. Independent determinants were found in logistic regression associated to HIV-1 incident infection: presence of HBsAg (odds-ratio (OR)=5.0; 95% CI 1.3-19.1) and race/ethnicity other than Caucasian (OR=4.0; 95% CI 1.1-14.7).
Conclusion: The avidity index methodology is simple and rapid, allowing the identification of recent infections. So far, there are no published national or international studies, allowing us to assess our annual projection of HIV-1 seroincidence, due to the recent introduction of this methodology.