How did RBM produce its most recent estimates on the burden of malaria worldwide?
Epidemiologist Eline Korenromp, whose work has been instrumental for producing the latest figures, explains
Q: How many cases of malaria occur each year worldwide?
A: Our estimate is 350 to 500 million cases, of which 270-400 million are falciparum malaria — the most severe form of the disease. Around 70% of the falciparum burden is estimated to occur in Africa and around 20% in South-East Asia. We are still in the process of refining these estimates.
Q: How does WHO produce this estimate?
A: We start with a map of populations at risk for malaria, which is produced with an outside research group. The map is created through two sets of data: reports by countries about where they believe malaria occurs and data regarding the intensity of malaria transmission within these risk areas. Using data from community-based research studies, we then estimate the frequency of malaria episodes occurring in population groups in each region. We combine these data to estimate the number of malaria cases occurring in each country annually. Last, country-level estimates are adjusted to reflect local coverage of insecticide-treated mosquito nets and indoor residual spraying, which reduce malaria. Our methods are reviewed by a group of independent technical experts.
Q: Why can't WHO provide more precise figures?
A: The best way to produce an accurate figure for malaria cases each year would be to obtain a reliable report every time anyone became ill with the disease. In most countries, those data are not available. That is why we rely above all on research studies in selected areas. It is uncertain, however, whether these studies are truly representative of an entire region.
In addition, our second source—the global risk map—may not be 100% accurate for all countries, because the intensity of malaria transmission fluctuates in response to environmental changes, development and malaria control measures.
Our estimate is presented as a range to allow for a margin of error.
Q: It seems the number of malaria cases has gone up. Why is that?
A: It is true that the new estimates are higher than 1998 WHO estimates. It is likely the increase is due mainly to improvements in the estimation method.
Q: Have there been any regional shifts?
A: South-East Asia now has a higher proportion of malaria cases than in previous WHO estimates. This difference mainly reflects an improvement in the methods and data sources used for the current estimations.
Q: When will firm estimates be available?
A: Final WHO/RBM country-level estimates will be made available on the MERG website in mid-2005. These will regularly be updated, based on changes in areas of transmission risk and increases in coverage of RBM interventions demonstrated in countries, to allow tracking of progress towards MDG and RBM goals. We will continue to work with countries to improve the quality of input data.
Q: If it will be possible to halve the malaria burden by 2010 as proposed by the RBM Partnership, how will this achievement be documented?
A: For Africa, the key indicator is the reduction in deaths from all causes in children under five years of age. This is because in Africa, malaria causes an estimated 18% of all child deaths, and in addition contributes indirectly to many more deaths through synergy with other illnesses. Death trends will be evaluated concurrently with data on increases in the number of people receiving effective prevention and treatment.
Outside Africa, there are fewer malaria deaths and the disease occurs across all age groups. Because a higher proportion of malaria cases are seen in formal health facilities, data coming from those facilities tends to be more useful for estimating disease trends. But as in Africa, it is also important to measure how many people are receiving malaria prevention and treatment.
In May 2005, the World Health Organization and UNICEF will launch the World Malaria Report 2005, a comprehensive overview of the status of malaria worldwide and countries’ progress controlling it through effective treatment and prevention.