METHODS
Analytic overview
A model-based cost-effectiveness analysis was conducted to estimate the
ratio of incremental costs and health benefits between SCIT + ICS
(intervention strategy) versus ICS (comparator strategy). A hypothetical
cohort (1,000 patients per strategy) of pediatric patients (8-year-old
at baseline) with a diagnosis of moderate persistent AA (sensitized to
HDM with clinically relevant symptoms) without AR was defined as the
target population to be simulated in the base case scenario. The
SCIT+ICS strategy consisted of a monthly administration scheme for three
years (a period in which the effect of SCIT is expected to be perceived)
followed by ICS within a 10-year time horizon (13). The comparator
strategy consisted in treatment with ICS + symptomatic medications
during the overall time horizon. The 10-year time horizon was defined as
a period in which the differences in long-term effects of SCIT+ ICS vs
ICS in health outcomes and costs would be observed. Different scenario
analyses were performed considering allergic rhinitis (AR) as a comorbid
condition in the base-case population and a cohort of adult patients (18
years old at baseline) with and without AR to evaluate potential
differences in cost-effectiveness estimations. The perspective of the
health care system was adopted, and only direct-medical costs were used.
Benefits associated to the evaluated strategies were expressed as
quality-adjusted life-years (QALYs). Costs and QALYs were discounted at
5% per year according to the Colombian guidelines for conducting
economic evaluations (14). An exchange rate of $3,250 Colombian pesos
(COP) per one American dollar (USD) was used to convert from 2018 COP to
USD. A willingness-to-pay threshold (WTP) of $18,125 USD (i.e., three
per capita Gross Domestic Product (GDP) of Colombia) per additional QALY
was defined as the criteria for evaluating cost-effectiveness (14). The
decision model was built using MS Excel 2018 (Microsoft, Redmond, WA).
Measures of effectiveness
The effect of the evaluated therapies was measured in terms of reduction
in the probability of a moderate or severe exacerbation (as an event of
emergency department visit – ED, or hospitalization, respectively), and
the reduction or discontinuation of asthma medications (as an indicators
of reduced disease severity and risk of exacerbations) (15). These
outcomes reflect an improvement of health-related quality of life and a
reduction in costs, and are thus considered robust metrics for relating
clinical effectiveness to costs and QALYs (9,16,17).
Model structure
A Markov model was developed based on the stepwise approach proposed by
the Global Strategy for Asthma Management and Prevention of the Global
Initiative of Asthma (GINA) in which treatment steps are associated to
different levels of asthma severity (18) (Figure 1). This conceptual
structure was used to create a flexible and reproducible model
accounting for relevant outcomes in clinical practice such as ED visits,
hospitalizations and medication-step down that have not been widely used
in previous HDM SCIT cost-effectiveness studies despite its recognized
relevance for policy makers and clinicians (17,19). Furthermore, this
model can be used to account for the different recommended treatment
schemes within each GINA Step according to the target population to be
simulated (12,18).