Projecting the long-term economic benefits of reducing Shigella-attributable linear growth faltering with a potential vaccine: a modelling study

Abstract

Linear growth is an important outcome of child development with implications for economic productivity. Enteric infections, particularly Shigella, have been linked to linear growth faltering (LGF). However, benefits from potential reductions in LGF are rarely included in economic analyses of enteric infections. We aimed to quantify the economic benefits of vaccination related to reduced Shigella-attributable disease and associated LGF compared with the net costs of a vaccine programme. In this benefit-cost analysis, we modelled productivity benefits in 102 low-income and middle-income countries that had recent stunting estimates available, at least one Shigella-attributable death annually, and available economic data, particularly on gross national income and growth rate projections. We modelled benefits strictly related to linear growth improvements and no other benefits associated with reducing diarrhoeal burden. The effect size in each country was calculated as shifts in height-for-age Z score (HAZ), representing population average changes for preventing Shigella-attributable less-severe diarrhoea and moderate-to-severe diarrhoea separately for children younger than 5 years. Benefits data were calculated per country and combined with estimated net costs of the vaccine programme in the form of benefit-cost ratios (BCRs); BCRs above parity, or $1 in benefits per $1 in costs (with a 10% margin representing borderline results: 1·10:1), were considered cost-beneficial. Countries were aggregated for analysis by WHO region, World Bank income category, and eligibility for support from Gavi, the Vaccine Alliance. In the base-case scenario, all regions exhibited cost-beneficial results, with the South-East Asia region and Gavi-eligible countries exhibiting the highest BCRs (21·67 for the South-East Asia region and 14·45 for Gavi-eligible countries), and the Eastern Mediterranean region yielding the lowest BCRs (2·90). All regions exhibited cost-beneficial results from vaccination, except in more conservative scenarios (eg, those assuming early retirement ages and higher discount rates). Our findings were sensitive to assumed returns for increased height, assumptions about vaccine efficacy against linear growth detriments, the anticipated shift in HAZ, and discount rate. Incorporating the productivity benefits of LGF reduction into existing cost-effectiveness estimates resulted in longer-term cost-savings in nearly all regions. LGF is a secondary outcome of Shigella infection and reduction in LGF is not often quantified as a health or economic benefit of vaccination. However, even under conservative assumptions, a Shigella vaccine only moderately effective against LGF could pay for itself from productivity gains alone in some regions. We recommend that LGF be considered in future models assessing the economic and health impacts of interventions preventing enteric infections. Further research is needed on vaccine efficacy against LGF to inform such models. Bill & Melinda Gates Foundation, Wellcome Trust.