Academic journal article Bulletin of the World Health Organization

Identifying High-Risk Areas for Sporadic Measles Outbreaks: Lessons from South Africa/Identifier Les Zones a Haut Risque D'epidemies Sporadiques De Rougeole: Les Lecons De l'Afrique Du Sud/Identificacion De Las Areas Con Riesgo Alto De Sufrir Brotes Esporadicos De Sarampion: Lecciones Desde Sudafrica

Academic journal article Bulletin of the World Health Organization

Identifying High-Risk Areas for Sporadic Measles Outbreaks: Lessons from South Africa/Identifier Les Zones a Haut Risque D'epidemies Sporadiques De Rougeole: Les Lecons De l'Afrique Du Sud/Identificacion De Las Areas Con Riesgo Alto De Sufrir Brotes Esporadicos De Sarampion: Lecciones Desde Sudafrica

Article excerpt

Introduction

Measles is highly contagious and the transmission intensity (1) of the measles virus exceeds that of most human pathogens. (2) Despite progress in controlling measles since 2000, the disease remains endemic in many countries and killed over 164 000 children worldwide in 2008. (1) Deaths also occur during outbreaks in areas where the disease is no longer endemic. (3)

Measles vaccination is highly effective, safe and relatively cost-effective (4) and has interrupted measles transmission in most parts of the world, (5) although high-risk areas remain in Africa and southern Asia. Immunization coverage is a key indicator for monitoring health sector performance and progress towards Millennium Development Goal 4 (i.e. reducing child mortality). (6) Measles is difficult to control and eliminate and more than 90% of the population must be immune to interrupt transmission and prevent outbreaks. (7-11) Maintaining adequate vaccine coverage throughout countries is a global problem and, in 2010 and 2011, outbreaks occurred even in the developed world. (4) Moreover, since the efficacy of the first vaccine dose is thought to be around 85%, (12,13) non-responders will add to the pool of susceptible children. In addition, achieving a high level of population immunity may be difficult in areas such as sub-Saharan Africa where the prevalence of human immunodeficiency virus (HIV) infection is high because infected children are less likely to respond to vaccination or maintain a protective antibody level or may acquire lower antibody levels from an HIV-positive (HIV+) mother. (14-16) Vaccination may need to be repeated more frequently in these areas. (16)

Since 1995, several African countries have launched initiatives to eliminate measles, as recommended by the World Health Organization (WHO). (17,18) 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. (17)

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. At present, measles vaccination is mandatory and provided in the public sector, whereas combination vaccination against measles, mumps and rubella is available in the private sector at a cost. The South African National Department of Health introduced case-based measles surveillance in 1998 with laboratory support from the National Institute for Communicable Diseases.

A large 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 18 000 cases recorded. (4) The transient trough in cases following a large outbreak, which increases herd immunity, affords planners an opportunity to investigate the epidemic and refine prevention strategies. (4) Spatial analysis has become an important epidemiological tool for detecting and predicting patterns of disease spread. (19) In situations in which population-wide interventions are too expensive or are inefficient because they dilute available resources, the effectiveness of vaccination is increased by targeting high-risk areas, such as those implicated in measles transmission and those where vaccination coverage varies. (20,21)

The objectives of this study were: (i) to investigate the pattern of measles spread in the South African outbreak between 2009 and 2011; (ii) to identify associations between the outbreak and population density, HIV prevalence and preceding lapses in measles vaccination; and (iii) to develop a model for identifying areas at a high risk for future large-scale sporadic outbreaks that can be used in prevention. …

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