Using mathematical modelling to investigate the plausibility of attributing observed antenatal clinic declines to a female sex worker intervention in Karnataka state, India

To determine whether the 32% and 52% decline in ANC HIV prevalence among female antenatal clinic (ANC) attenders, observed in Avahan districts between 2004 and 2006, and 2007 respectively, in the state of Karnataka could be due to a HIV preventive intervention targeted at female sex workers and their clients. An exhaustive sensitivity analysis, based on an age and parity structured mathematical model of HIV transmission in a general and ANC population, was undertaken to estimate intervention impact in different concentrated HIV epidemics representative of those in Karnataka districts. To assess if the large reduction in ANC HIV prevalence could be solely due to the intervention, we simulated a very optimistic intervention. If 100% of FSWs were reached and condom use between clients and FSWs increased instantaneously to over 80% of sex acts, the expected intervention decline (50th, (10th, 90th) percentiles) among the overall and 15-19 year old ANC population after three years of intense intervention activity was 21% (14%, 27%) and 27% (19%, 35%); with a predicted time required to produce a 30% intervention decline being approximately 5 (4.0, 6.4) and approximately 3.6 (2.8, 4.8) years, respectively. To achieve this magnitude of decline, the client and FSW HIV prevalence needed to decrease by 33% (28%, 38%) and 44% (38%, 50%), respectively, after three years. Despite the optimistic prevention parameters assumed, our results suggest that the large observed changes in ANC HIV prevalence are very unlikely to already be entirely caused by the FSW targeted intervention. Interpretation of HIV trends in ANC populations should involve triangulation of observed biological and behavioural trends in high-risk groups, modeling studies and documentation of possible sources of bias.