IDENTIFYING HIGH-RISK AREAS FOR SPORADIC MEASLES OUTBREAKS: LESSONS FROM SOUTH AFRICA

Tuesday, 14th of January 2014

[source]Bulletin of the World Health Organization[|source]

Since 1995, several African countries have launched initiatives to eliminate measles, as recommended by the World Health Organization (WHO). These initiatives aim to: (i) achieve a coverage of 90% or more for the first vaccine dose; (ii) identify areas with low coverage and high-risk areas for “mop-up” immunization of children who missed vaccination; and (iii) enhance case-based measles surveillance with laboratory support. In South Africa, first and second measles vaccine doses have been given free of charge to children aged 9 and 18 months since April 2009 as part of the WHO Expanded Program on Immunization. However, alarge outbreak of measles involving around 1700 cases occurred in South Africa between 2003 and 2005 following its introduction from Mozambique. More recently, there was an outbreak between 2009 and 2011, with over 18000 cases recorded.

In this report, the authors investigated the pattern of measles spread in the South African outbreak between 2009 and 2011; identified associations between the outbreak and population density, HIV prevalence and preceding lapses in measles vaccination; and developed a model for identifying areas at a high risk for future large-scale sporadic outbreaks that can be used in prevention. More details are available at:  http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3590621/

Abstract

Objective: To develop a model for identifying areas at high risk for sporadic measles outbreaks based on an analysis of factors associated with a national outbreak in South Africa between 2009 and 2011.

Methods: Data on cases occurring before and during the national outbreak were obtained from the South African measles surveillance programme, and data on measles immunization and population size, from the District Health Information System. A Bayesian hierarchical Poisson model was used to investigate the association between the risk of measles in infants in a district and first-dose vaccination coverage, population density, background prevalence of human immunodeficiency virus (HIV) infection and expected failure of seroconversion. Model projections were used to identify emerging high-risk areas in 2012.

Findings: A clear spatial pattern of high-risk areas was noted, with many interconnected (i.e. neighbouring) areas. An increased risk of measles outbreak was significantly associated with both the preceding build-up of a susceptible population and population density. The risk was also elevated when more than 20% of infants in a populous area had missed a first vaccine dose. The model was able to identify areas at high risk of experiencing a measles outbreak in 2012 and where additional preventive measures could be undertaken.

Conclusion: The South African measles outbreak was associated with the build-up of a susceptible population (owing to poor vaccine coverage), high prevalence of HIV infection and high population density. The predictive model developed could be applied to other settings susceptible to sporadic outbreaks of measles and other vaccine-preventable diseases.