The estimated cost-effectiveness of a Shigella vaccine in children under five in Ethiopia
Presenter: Katherine Rosettie
Co-authors: James Collins, Michelle Park, Abraham D. Flaxman, Ibrahim Khalil, Kelly Compton, Paola Pedroza, Ali H. Mokdad, Marcia R. Weaver
Poster file: [download]
Shigella is the second leading cause of diarrhea mortality in children under five globally, and was responsible for approximately 8,500,000 disability-adjusted-life-years (DALYs) in 2017 in this age group. Shigella incidence peaks among children one to four years, and Shigella trials are being designed to test a vaccine with doses administered in the first and second year of life. We aim to estimate the cost-effectiveness of a Shigella vaccine in Ethiopia, given the country’s mid-range diarrhea mortality and increasing vaccine coverage. Our objectives are to answer the following four questions in Ethiopia: (1) What is the effect of adding a primary Shigella vaccine series at 9 and 12 months to the routine immunization schedule?; (2) What is the effect of adding a booster dose at 15 months to the primary series?; (3) What is the effect of a booster dose at 18 versus 15 months?; and (4) What is the effect of starting the primary series at 6 months versus 9 months? We used an open-source, individual-based predictive simulation framework developed at the Institute for Health Metrics and Evaluation called Vivarium to estimate the cost-effectiveness of a Shigella vaccine under different vaccine schedules. Vivarium leverages the most recent estimates from the Global Burden of Disease (GBD) study to model interactions between diarrhea risk factors, diarrhea incidence, and mortality by age, sex, year, and location. We used a time horizon of 15 years (2025-2040) with an initial population of 32,000 simulants ages 0-5 years and an open-cohort structure wherein newborns entered the model each time step (1 day). We conservatively assumed 50% vaccine efficacy. Cost estimates were based on summary estimates of routine immunizations in low-income countries. We used GBD-reported annual measles and DTP3 vaccines coverage in Ethiopia, which correspond to the timing of the Shigella vaccine in the routine immunization schedule, as a proxy for Shigella vaccine coverage. We quantified parameter uncertainty for our estimates by using the Monte Carlo method whereby we drew 500 values from the distributions of each input parameter. All results were scaled to the national level. Compared to a baseline scenario with no Shigella vaccine, adding two doses of the vaccine at 9 and 12 months would avert 101,000 DALYs, with an ICER of $2110/DALY averted. Compared to a primary series at 9 and 12 months, adding a booster dose at 15 months would avert 157,000 DALYs save US$16.5 million. Compared to booster at 15 months, a later booster at 18 months would be cost-saving (20,000 DALYs averted; US$20.56 million saved). Finally, when comparing an earlier primary series at 6 and 9 months to a later primary series at 9 and 12 months, the earlier series would be cost-saving with 62,000 DALYs averted and US$44.3 million saved. This suggests a primary series at 6 and 9 months with a booster 15 months is the most optimal Shigella vaccine schedule, yet we estimated that all four vaccine schedules would be cost-saving in Ethiopia and would result in a reduction in the Shigella burden.