Transmission dynamics and vaccination strategies for Crimean-Congo haemorrhagic fever virus in Afghanistan: A modelling study

Abstract

Background Crimean-Congo haemorrhagic fever virus (CCHFV) is a highly pathogenic virus for which a safe and effective vaccine is not yet available, despite being considered a priority emerging pathogen. Understanding transmission patterns and the use of potential effective vaccines are central elements of the future plan against this infection. Methods We developed a series of models of transmission amongst livestock, and spillover infection into humans. We use real-world human and animal data from a CCHFV endemic area in Afghanistan (Herat) to calibrate our models. We assess the value of environmental drivers as proxy indicators of vector activity, and select the best model using deviance information criteria. Finally we assess the impact of vaccination by simulating campaigns targeted to humans or livestock, and to high-risk subpopulations (i.e, farmers). Findings Saturation deficit is the indicator that better explains tick activity trends in Herat. Recent increments in reported CCHFV cases in this area are more likely explained by increased surveillance capacity instead of changes in the background transmission dynamics. Modelling suggests that clinical cases only represent 31% (95% CrI 28%-33%) of total infections in this area. Vaccination campaigns targeting humans would result in a much larger impact than livestock vaccination (266 vs 31 clinical cases averted respectively) and a more efficient option when assessed in courses per case averted (35 vs 431 respectively). Targeted vaccination of farmers is impactful and more efficient, resulting in 19 courses per case averted (95% CrI 7–62) compared to targeting the general population (35 courses 95% CrI 16–107) Conclusions CCHFV is endemic in Herat, and transmission cycles are well predicted by environmental drivers like saturation deficit. Vaccinating humans is likely to be more efficient and impactful than animals, and importantly targeted interventions to high risk groups like farmers can offer a more efficient approach to vaccine roll-out.

Vaccine efficacy trials for Crimean-Congo haemorrhagic fever: insights from modelling different epidemiological settings

“…Transmission models for CCHFV amongst livestock and humans were calibrated to data from endemic areas in Afghanistan, South Africa and Turkey (Figs S1-S3). As reported before, saturation deficit is the environmental driver that best describes seasonality and tick activity in Afghanistan [10]. In Turkey and South Africa, soil temperature is a better surrogate marker, according to DIC (See these patterns, our selected optimal times for starting trial recruitment in each setting appear to be May, April and October in Afghanistan, Turkey and South Africa, respectively.…”

“…Time series for these drivers was gathered at the province level for each country. These models and the role of environmental drivers are explained in detail elsewhere [22].…”

“…Findings from our previous analysis of the transmission dynamics of CCHFV in an endemic region in Afghanistan [10] suggest that a future vaccination campaign targeted to human groups at high risk of infection should be preferred over animal vaccination in this setting.…”