Political boundaries, e.g. countries, need not represent a single epidemiologically relevant population. Variation within a country with respect to demography and health system performance combine to generate a heterogeneous landscape for disease transmission. Outbreak response efforts that average across this variation to impose a one-size-fits-all policy may over-allocate effort in low-risk areas while underserving high-risk area.
To illustrate the impact of transmission variability across a landscape, we analyze a 2010 measles outbreak in Malawi, with over 135,000 reported cases. A nonselective outbreak response vaccination campaign with uniform age targets vaccinated over 3 millions children in 8 districts. We analyze both the outbreak time series and age-distribution of cases to identify the minimal local vaccination targets necessary to reduce the basic reproductive number below 1.
The fast pace of measles epidemiology mandates the development of rapid response strategies. High coverage, wide age-target vaccination campaigns can hit a point of diminishing returns in highly-immune communities while imposing costs on other communities by delaying arrival of medical staff. We therefore develop a method for defining a minimal sufficient response and propose that all communities be targeted with an intervention as close to the minimum as practical, permitting rapid redeployment of staff and resources.
Results/Conclusions
When analyzed at the local scale, the course of the 2010 measles outbreak in Malawi showed substantial variation across the landscape. Though RE was 1.8 on average, 90% of local estimates ranged from 1.3 to 2.6, suggesting strong differences in the required coverage levels of reactive vaccination campaigns that would to reduce RE below 1 and halt transmission. Similarly, the median age of infected individuals ranged from 5 to 10, implying differences in the required age range of vaccination campaigns.
We demonstrate that the strategy of allocating outbreak response effort equally across space can result in an inequitable outbreak risk as measured by the effective reproductive number following a mass vaccination campaign. Setting a single national policy without considering heterogeneities and the differing dynamics across scales would have allocated more vaccine than necessary in some districts, while leaving other portions of the country, representing 84% of Malawi’s population, underserved.