Abstract
BackgroundThis paper compares cost-effectiveness results from two models of maternal immunization to prevent pertussis in infants in Brazil, one static, one dynamic, to explore when static models are adequate for public health decisions and when the extra effort required by dynamic models is worthwhile. MethodsWe defined two scenarios to explore key differences between static and dynamic models, herd immunity and time horizon. Scenario 1 evaluates the incremental cost/DALY of maternal acellular pertussis (aP) immunization as routine infant vaccination coverage ranges from low/moderate up to, and above, the threshold at which herd immunity begins to eliminate pertussis. Scenario 2 compares cost-effectiveness estimates over the models’ different time horizons. Maternal vaccine prices of $9.55/dose (base case) and $1/dose were evaluated. ResultsThe dynamic model shows that maternal immunization could be cost-saving as well as life-saving at low levels of infant vaccination coverage. When infant coverage reaches the threshold range (90–95%), it is expensive: the dynamic model estimates that maternal immunization costs $2 million/DALY at infant coverage > 95% and maternal vaccine price of $9.55/dose; at $1/dose, cost/DALY is $200,000. By contrast, the static model estimates costs/DALY only modestly higher at high than at low infant coverage. When the models’ estimates over their different time horizons are compared at infant coverage < 90–95%, their projections fall in the same range. ConclusionsStatic models may serve to explore an intervention’s cost-effectiveness against infectious disease: the direction and principal drivers of change were the same in both models. When, however, an intervention too small to have significant herd immunity effects itself, such as maternal aP immunization, takes place against a background of vaccination in the rest of the population, a dynamic model is crucial to accurate estimates of cost-effectiveness. This finding is particularly important in the context of widely varying routine infant vaccination rates globally. Clinical Trial registryClinical Trial registry name and registration number: Not applicable.
Highlights
The special characteristics of many infectious diseases – transmission from one individual to others, natural immunity after⇑ Corresponding author.recovery from the disease, and the herd immunity that protects susceptible individuals when other people in the population are immune – are best captured by dynamic transmission simulation models
Dynamic model Like the static model, the dynamic model estimates that maternal immunization saves lives, usually at additional cost, it can be cost-saving in some conditions, especially when the price of the maternal vaccine is very low
The pattern of cost/disability-adjusted life-year (DALY) projected by the dynamic model as routine infant vaccination coverage rises, stands in stark contrast to that of the static model
Summary
The special characteristics of many infectious diseases – transmission from one individual to others, natural immunity after⇑ Corresponding author.recovery from the disease, and the herd immunity that protects susceptible individuals when other people in the population are immune – are best captured by dynamic transmission simulation models. Scenario 1 evaluates the incremental cost/DALY of maternal acellular pertussis (aP) immunization as routine infant vaccination coverage ranges from low/moderate up to, and above, the threshold at which herd immunity begins to eliminate pertussis. An intervention too small to have significant herd immunity effects itself, such as maternal aP immunization, takes place against a background of vaccination in the rest of the population, a dynamic model is crucial to accurate estimates of cost-effectiveness. This finding is important in the context of widely varying routine infant vaccination rates globally.
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