Spraying insecticides indoors offers children no additional protection from malaria when bed nets are used, a study said on Tuesday, as malaria cases and deaths worldwide continue to fall.
A study by medical journal The Lancet said donors should invest their limited resources on additional bed nets as the most cost-effective solution to tackling malaria, costing an average of $2.20 per person compared to $6.70 for insecticide.
"High bed net use is sufficient to protect people against malaria in areas that have low or moderate levels of malaria," lead author Steve Lindsay said in a statement.
Malaria, a mosquito-borne parasitic disease, kills more than 600,000 people a year, and most victims are children under five living in the poorest parts of sub-Saharan Africa.
The study coincided with the launch of the World Health Organisation's (WHO) annual World Malaria Report, which said the number of global malaria deaths fell by 47 percent between 2000 and 2013, with malaria cases also steadily declining, due to improved access to testing, treatment and bed nets.
In Africa, the number of people infected fell to 128 million in 2013 from 173 million in 2000, despite a 43 percent increase in the African population living in malaria transmission areas.
Insect nets and indoor spraying of insecticide have been proven to reduce the number of malaria deaths, The Lancet said.
Last year almost half of people at risk of malaria in sub-Saharan Africa had access to bed nets, compared to just three percent in 2004, according to the WHO, while 55 million people, seven percent of the population at risk, lived in households that were regularly sprayed.
Yet there had been little research on whether combining these interventions would give better results.
The study said that clinical trials in Gambia, involving almost 8,000 children across 70 villages, showed that spraying insecticides indoors offered no additional protection from malaria when bed nets were used.
Where bed nets are unavailable or in short supply, indoor insecticide spraying should be considered, it said.
David Conway, Professor of Biology at the London School of Hygiene and Tropical Medicine, said the study indicated that it would be hard to reduce malaria further by adding more preventative measures indoors.
It was likely that some of the remaining transmission of malaria was due to mosquitoes biting people outside in the evening, Conway said.