Study population

HIV is a legally notifiable disease in Singapore. The HIV notification system is supplemented with additional information obtained through review of medical case notes and interviews with the cases. The information collected on all HIV cases includes socio-demographic characteristics, the first CD4 count, mode of detection and exposure factors.

To better monitor the dynamics of HIV transmission in Singapore, the National Public Health Laboratory (NPHL) commenced detection of RHI on residual plasma samples from HIV cases since 2013, as part of the National HIV Molecular Surveillance Programme under the Infectious Diseases Act (IDA) [18]. The proportion of RHI in newly-diagnosed HIV cases can be estimated using serological assays to measure the level of HIV-1-specific antibodies out of total immunoglobulin (IgG), which increases with time since infection [19, 20]. To further increase the specificity of the result, a recent infection testing algorithm (RITA), taking CD4 counts and supplementary clinical information into consideration to classify an HIV infection as recent or long-term, was applied as recommended by the World Health Organization (WHO) guidelines [21]. The main advantage of such an approach in determining the evidence of RHI is that a single sample can be taken at the time of HIV diagnosis without the need for follow-up, unlike with cohort studies.

Test results from NPHL were linked to the National HIV Registry using unique personal identifiers tagged to the samples. Personal identifiers were permanently removed from the merged database prior to statistical analysis.

Residual plasma samples of individuals whose HIV infection had been confirmed by Western blot assay were collected from public acute-care tertiary hospitals for the National HIV Molecular Surveillance Programme. We excluded HIV cases notified to the National HIV Registry from this study if they met one or more of the following criteria: unavailability of CD4 count at diagnosis; presence of an AIDS-defining illness at the time of diagnosis; commencement of ART prior to specimen collection. The testing for RHI was confined to plasma samples drawn from treatment-naïve individuals within 12 months of HIV diagnosis in 2013–2017.