Dynamics of the HIV outbreak and response in Scott County, IN, USA, 2011-15: a modelling study

In November, 2014, a cluster of HIV infections was detected among people who inject drugs in Scott County, IN, USA, with 215 HIV infections eventually attributed to the outbreak. This study examines whether earlier implementation of a public health response could have reduced the scale of the outbreak.

In this modelling study, we derived weekly case data from the HIV outbreak in Scott County, IN, and on the uptake of HIV testing, treatment, and prevention services from publicly available reports from the US Centers for Disease Control and Prevention (CDC) and researchers from Indiana. Our primary objective was to determine if an earlier response to the outbreak could have had an effect on the number of people infected. We computed upper and lower bounds for cumulative HIV incidence by digitally extracting data from published images from a CDC study using Bio-Rad avidity incidence testing to estimate the recency of each transmission event. We constructed a generalisation of the susceptible-infectious-removed model to capture the transmission dynamics of the HIV outbreak. We computed non-parametric interval estimates of the number of individuals with an undiagnosed HIV infection, the case-finding rate per undiagnosed HIV infection, and model-based bounds for the HIV transmission rate throughout the epidemic. We used these models to assess the potential effect if the same intervention had begun at two key timepoints earlier than the actual date of the initiation of efforts to control the outbreak.

The upper bound for undiagnosed HIV infections in Scott County peaked at 126 around Jan 10, 2015, over 2 months before the Governor of Indiana declared a public health emergency on March 26, 2015. Applying the observed case-finding rate scale-up to earlier intervention times suggests that an earlier public health response could have substantially reduced the total number of HIV infections (estimated to have been 183-184 infections by Aug 11, 2015). Initiation of a response on Jan 1, 2013, could have suppressed the number of infections to 56 or fewer, averting at least 127 infections; whereas an intervention on April 1, 2011, could have reduced the number of infections to ten or fewer, averting at least 173 infections.

Early and robust surveillance efforts and case finding alone could reduce nascent epidemics. Ensuring access to HIV services and harm-reduction interventions could further reduce the likelihood of outbreaks, and substantially mitigate their severity and scope.

US National Institute on Drug Abuse, US National Institutes of Mental Health, US National Institutes of Health Big Data to Knowledge programme, and the US National Institutes of Health.