Hybrid surveillance for incidence of severe enteric fever in India

Under the Surveillance for enteric fever in India project done between 2017-20, urban areas of Vellore and Chandigarh were found to have a high burden of typhoid fever. While the incidence of typhoid was estimated as 12 per 1000 person-years (PY) in Vellore using a cohort-based approach, Chandigarh noted comparable estimates of about 10 cases per 1000 PY.Since the incidence of typhoid fever varies considerably over time and across settings, continued surveillance is necessary to establish the burden. Cohort studies, while offering good approaches to uncover ongoing transmission, are not optimal for establishing the severity of disease as the active surveillance results in early treatment, and thus bias to milder disease. Hybrid surveillance is an alternative approach that estimates the incidence of more severe enteric fever by combining hospital-based surveillance with healthcare utilization by the community and are significantly less resource-intensive. Using this approach, we will prospectively undertake blood culture surveillance for febrile admissions at hospitals that account for at least 60% of all febrile admissions in the reference population. Health care utilization pattern of the study population for fever-related hospitalizations during the study period will be captured using bi-annual healthcare utilization surveys (HCUS) as Covid-19 may influence healthcare utilization over time.

We will estimate the crude incidence of blood-culture-confirmed enteric fever by dividing the number of cases of enteric fever identified in the population at risk. These crude incidence estimates will be adjusted for healthcare utilization patterns observed in the bi-annual HCUS, the proportion of febrile admissions that received blood culture and the diagnostic accuracy blood culture (sensitivity), to arrive at the adjusted incidence of enteric fever. In addition, we will characterize the severity of enteric fever presenting at these facilities and estimate the proportion of enteric fevers that present to healthcare facilities but are managed as ambulatory patients