Malaria kills more than 1.2 million people a year, nearly 50 percent more than previously thought, and inflicts a high toll among adults and older children and not just toddlers, a new investigation says.

But there is also good news: deaths from the mosquito-borne disease have in fact been falling sharply thanks to access to better drugs and insecticide-treated nets.

Published in The Lancet on Friday, the study by the Institute of Health Metrics and Evaluation (IHME) at the University of Washington, Seattle, says malaria killed at least 1.2 million people worldwide in 2010.

The estimate will be a likely shock for health policymakers. Only last September the UN-backed Roll Back Malaria (RBM) calculated mortality in 2009 at 781,000.

The higher figure, say the US researchers, derives from wider and more reliable data, including use of a technique called "verbal autopsy".

Under this, investigators interview relatives of someone who has recently died in order to help pinpoint the cause of death. In many poor countries which lack medical infrastructure, mortality is often poorly probed or misidentified.

The new study skewers the belief that the overwhelming majority of malaria deaths occur among the under-fives.

In 2010, more than 78,000 children aged five to 14, and more than 445,000 aged 15 or older, died of malaria, together accounting for 42 percent of the total.

"You learn in medical school that people exposed to malaria as children develop immunity and rarely die from malaria as adults," said lead researcher Christopher Murray.

"What we have found in hospital records, death records, surveys and other sources shows that just is not the case."

From 1985, says the paper, malaria deaths grew every year, peaking at 1.8 million in 2004.

But from 2004, the toll fell every year. Between 2007 and 2010, the decline has been particularly acute -- more than seven percent every year -- and the big beneficiary has been Africa.

Among the stars are Tanzania and Zambia, which saw deaths fall by more than 30 percent between 2004 and 2010.

The source of the decline lies in the increased use of artemisinin drugs, replacing medications to which the malaria parasite has become resistant, and the widening distribution of insecticide-treated bed nets, says the probe.

The major players in this campaign are the Bill and Melinda Gates Foundation, RBM and the Global Fund to Fight AIDS, Malaria and Tuberculosis. Funding rose from less than 250 million dollars annually in 2001 to more than two billion in 2009.

"We have seen a huge increase in both funding and in policy attention given to malaria over the past decade, and it's having real impact," said Alan Lopez, a population health expert at the University of Queensland in Australia, who was a member of the research team.

The study warns of the dangers if the momentum is lost, especially in the shortfall of support for the Global Fund.

"There has been a rapid decrease in malaria mortality in Africa because of the scaling up of control activities supported by international donors," it says.

"Donor support, however, needs to be increased if malaria elimination and eradication and broader health and development goals are to be met."

In last September's estimate, RBM said mortality from malaria fell from 984,000 in 2000 to 781,000 in 2009, a decline of 38 percent if the world's population growth over this period is factored in.

The new study was funded by the Gates Foundation; however, the IHME is an independent research institution and The Lancet is a peer-reviewed journal.

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