Table 1.
Model Parameters
| Parameter | Mean (95% confidence intervals) | Source |
|---|---|---|
| Transmission and natural history | ||
| Transmission rate | Varies by state, age group, urban-rural setting | Calibrated (see Supplementary Technical Appendix) |
| Symptomatic infections (% of total) | 10% (6%–21%) | Unpublished estimates from seroprevalence studies |
| Duration of infection | 20 d (12–30 d) | SEFI surveillance data, assume shedding lasts twice as long as symptoms based on data from challenge studies |
| Case fatality fraction | 0.18% (0.07%–0.36%) | SEFI surveillance data; details in Supplementary Technical Appendix |
| Percentage of infections that mount protective immune response (ie, transition from infected to recovered) | 50% (25%–75%) | Estimates from seroprevalence studies, vaccine trial surveillance data, and human challenge studies [25–28] |
| Duration of immunity against clinical infection | 20 y (15–30 y) | Assumed to be the same as duration of immunity from vaccination |
| Relative infectiousness of subclinical infections | 72% (44%–100%) | Control group data from Darton et al [29] |
| Relative risk of subclinical infection after recent infection | 63% (49%–93%) | Calculated from Gibani et al [27] |
| Carrier epidemiology | ||
| Percentage of infections that progress to carrier | 0.03%–1.2% (varies by age) | Woodward unpublished report, reported in Gibani et al [27]; Ames et al [30]. See Supplementary Technical Appendix. |
| Duration of carriage | 10 y (5–15 y) | Based on Ames et al [30]; Bhan et al [31], Gunn et al [32] |
| Relative infectiousness of carriers, compared with acute infections | 7.5% (5.5%–9.5%) | Calibrated estimate from Lo et al [17] |
| Vaccine characteristics | ||
| Vaccine efficacy | 82% (59%–92%) | Shakya et al [16] |
| Duration of immunity (TCV) | 20 y (15–30 y) | Calculations from seroconversion data from Lanh et al [20] and Bharat Biotech (unpublished data) |
| Duration of immunity (ViPS) | 4 y | Systematic review of vaccine trial data [33] |
| Vaccine coverage | ||
| Routine coverage | 47%–98% (varies by state and urban-rural setting) | Measles coverage data from the India National Family Health Survey IV [22] |
| Campaign coverage | 90% | Date et al [23]; alternatives explored in sensitivity analysis |
| School coverage | 90% | |
| Status quo coverage | 6% (4%–8%) | SEFI surveillance data |
| Vaccine costs per dose | ||
| Vaccine | $1.00 | Price announcements from the manufacturer [34] |
| Syringes and safety equipment | $0.031 | India Comprehensive Multi-Year Plans [46, 47] |
| Routine delivery—healthcare costs | $1.47 ($1.31–$1.62) | National Health System Cost Database for India [35] |
| Routine delivery—out of pocket and time costs | $0 | No incremental costs because delivery is alongside measles first dose |
| Campaign delivery—healthcare costs | $1.53 ($1.40–$4.23) | Date et al [23] |
| Campaign delivery—out of pocket and time costs | $0.53 ($0.00–$1.49) | Mogasale et al [36] |
| School-based delivery—healthcare costs | $1.16 ($0.58–$1.74) | Literature review of other school-based vaccination delivery costs [37–40] |
| School-based delivery—startup healthcare costs | $2.00 ($1.50–$2.50) | Literature review of other school-based vaccination delivery costs [37–40] |
| School delivery—out of pocket and time costs | $0 | No incremental costs because delivery is to children who are already in school |
| Cost of illness (for symptomatic infections) | ||
| Healthcare costs of illness—adult | $20 ($13–$47) | SEFI surveillance data (weighted average of hospitalized and nonhospitalized cases; see Supplementary Technical Appendix and Supplementary Table 2) |
| Healthcare costs of illness—pediatric | $25 ($13–$37) | |
| Out of pocket costs of illness | $10 ($7–$17) | |
| Productivity costs of illness | $56 ($52–$59) | |
| Quality of life and disability | ||
| Severe cases (% of symptomatic infections) | 16% (12%–20%) | SEFI surveillance data |
| Moderate cases (% of symptomatic infections) | 84% (80%–88%) | SEFI surveillance data |
| Ileal perforation (% of severe cases) | 2.5% (0.8%–5.1%) | SEFI surveillance data |
| Duration of symptoms (severe cases) | 11.5 d (10.0–13.0 d) | SEFI surveillance data |
| Duration of symptoms (moderate cases) | 9.2 d (8.8–9.5 d) | SEFI surveillance data |
| Duration of symptoms (severe cases with ileal perforation) | 13.4 d (9.6–17.1 d) | SEFI surveillance data |
| Disability weight (moderate typhoid) | 0.051 (0.032–0.074) | Roth et al [41] |
| Disability weight (severe typhoid) | 0.133 (0.088–0.190) | Roth et al [41] |
| Disability weight (typhoid with ileal perforation) | 0.324 (0.220–0.442) | Roth et al [41] |
| Demographics | ||
| Birth rate | Varies by state, urban-rural setting | India SRS bulletin 2019 [42] |
| All-cause mortality rate and life expectancy | Varies by state, age, urban-rural setting | India SRS life tables 2012–2016 [43] |
| Urban-rural migration rates | Varies by age | India Human Development Survey 2011–2012 [44] |
| Population size | Varies by state, age, urban-rural setting | India Census 2011 [45] |
Abbreviations: SEFI, Surveillance for Enteric Fever in India; SRS, Sample Registration System; TCV, typhoid conjugate vaccine; ViPS, Vi polysaccharide vaccine.