Economic evaluation of pneumococcal conjugate vaccination in The Gambia

Abstract

Background: Gambia is the second GAVI support-eligible country to introduce the 7-valent pneumococcal conjugate vaccine (PCV7), but a country-specific cost-effectiveness analysis of the vaccine is not available. Our objective was to assess the potential impact of PCVs of different valences in The Gambia.

Methods: We synthesized the best available epidemiological and cost data using a state-transition model to simulate the natural histories of various pneumococcal diseases. For the base-case, we estimated incremental cost (in 2005 US dollars) per disability-adjusted life year (DALY) averted under routine vaccination using PCV9 compared to no vaccination. We extended the base-case results for PCV9 to estimate the cost-effectiveness of PCV7, PCV10, and PCV13, each compared to no vaccination. To explore parameter uncertainty, we performed both deterministic and probabilistic sensitivity analyses. We also explored the impact of vaccine efficacy waning, herd immunity, and serotype replacement, as a part of the uncertainty analyses, by assuming alternative scenarios and extrapolating empirical results from different settings.

Results: Assuming 90% coverage, a program using a 9-valent PCV (PCV9) would prevent approximately 630 hospitalizations, 40 deaths, and 1000 DALYs, over the first 5 years of life of a birth cohort. Under base-case assumptions ($3.5 per vaccine), compared to no intervention, a PCV9 vaccination program would cost $670 per DALY averted in The Gambia. The corresponding values for PCV7, PCV10, and PCV13 were $910, $670, and $570 per DALY averted, respectively. Sensitivity analyses that explored the implications of the uncertain key parameters showed that model outcomes were most sensitive to vaccine price per dose, discount rate, case-fatality rate of primary endpoint pneumonia, and vaccine efficacy against primary endpoint pneumonia.

Conclusions: Based on the information available now, infant PCV vaccination would be expected to reduce pneumococcal diseases caused by S. pneumoniae in The Gambia. Assuming a cost-effectiveness threshold of three times GDP per capita, all PCVs examined would be cost-effective at the tentative Advance Market Commitment (AMC) price of $3.5 per dose. Because the cost-effectiveness of a PCV program could be affected by potential serotype replacement or herd immunity effects that may not be known until after a large scale introduction, type-specific surveillance and iterative evaluation will be critical.

Figure 1

Model schematic. This figure presents the schematic of the Markov model. Solid arrows represent transition probabilities between health states, which are differentiated depending on immunization status. Dashed arrows represent disease-specific deaths. Curved arrows represent that individuals stay in the same health states during the next cycle. Deaths due to other causes occur at every stage according to the background mortality rate for Gambian children but are not shown.

Figure 2

Selected model-predicted health outcomes. This figure presents the estimated numbers of cases of different epidemiological outcomes due to S. pneumoniae infection in the Gambia according to vaccine type, and compared to no vaccination.

Figure 3

Results of univariate sensitivity analysis. The tornadogram shows selected results of univariate sensitivity analysis for PCV9. The x-axis represents the range of the incremental cost-effectiveness ratios for vaccination using PCV7 when the base-case assumptions were varied over plausible ranges (as shown in the brackets). The vertical line represents the base case cost-effectiveness ratio of PCV7, $910 per DALY averted.

Figure 4

Deterministic sensitivity analysis: Cost-effectiveness of pneumococcal conjugate vaccines (PCVs) by vaccine price. This graph shows how cost-effectiveness of each type of PCVs varies as the unit price of vaccines are varied up to $10. The lower horizontal line indicates the threshold cost-effectiveness ratio based on Gambia's GDP per capita. The upper horizontal line indicates three times GDP per capita.

Figure 5

Probabilistic sensitivity analysis: Cost-effectiveness acceptability curves. This graph summarizes the results of a probabilistic sensitivity analysis from the societal perspective. The curve shows, for each type of PCVs, the probabilities that pneumococcal vaccination would be cost-effective at varying cost-effectiveness threshold ratios. For example, the probabilities that PCV7 would be cost-effective are 8% and 66% at cost-effectiveness thresholds of $360 (corresponding to The Gambia's GDP per capita) and $1,080 (corresponding to three times The Gambia's GDP per capita) per DALY averted, respectively. All PCVs would be considered 100% cost-effective with the threshold set at $2,400 per DALY averted.