Since the 1990s, bed nets impregnated with insecticides have been an invaluable tool for malaria prevention. Babies and young children who sleep under them are far less likely to die of the disease. But some scientists have worried this might increase the risk of contracting malaria later in life by preventing development of the immunity that protects older children and adults in areas where malaria is common. Now, an ambitious new study allays those fears. Thousands of people who slept under bed nets as youngsters in the late 1990s and early 2000s in Tanzania still have a significant survival advantage through young adulthood over peers who did not.
“It is a powerful demonstration of the long-lasting payoffs of protecting the health of young children,” says Pedro Alonso, director of the World Health Organization’s Global Malaria Programme, who was not involved in the new study. It’s also encouraging news for global health experts as they plan to roll out a new vaccine against malaria. That vaccine also aims to prevent the disease in the youngest, most vulnerable age groups, raising similar worries of a rebound effect.
Insecticide-treated bed nets, which prevent mosquitoes from biting people—and kill the insects on contact—were first systematically studied in the 1990s. Most studies only lasted a few years; the longest stretched just 7.5 years. Scientists from the Ifakara Health Institute (IFI) in Dar es Salaam, Tanzania, studied the nets’ effect on the health of 6706 babies and toddlers born between 1998 and 2000. By the end of the study, children who slept under the nets had a 27% increased chance of survival. (Because malaria leaves children more vulnerable to other diseases, researchers often measure “all-cause mortality” to assess the effects of malaria prevention measures.)
Twenty years later, the researchers wondered what had become of the children in the study, so they decided to track down as many of them as they could. The bed net study had been part of a larger health survey that built up extensive personal connections in the region, says Sigilbert Mrema, an IFI demographer who led the work with IFI epidemiologist Salim Abdulla. By asking community leaders, family members, neighbors, and other contacts, the researchers and their colleagues tracked down what happened to 89%, or 5983, of the children in the original study. That is “really remarkable,” Alonso says.
“People were really happy to be involved,” says Joanna Schellenberg, an epidemiologist at the London School of Hygiene & Tropical Medicine who helped lead both the original study and the follow-up.
The researchers found the survival advantage first seen in 2003 was still apparent 20 years later. Children who had slept under bed nets more than half the time in the original study had a 40% survival advantage over children who used them less than half the time, the researchers report today in The New England Journal of Medicine. And the protective effect of the nets was apparent even after they controlled for family income, parents’ education, and distance from a health care center.
The findings put to rest the worry that bed nets delay deaths rather than preventing them, says malaria researcher Thomas Eisele of Tulane University, who wasn’t involved in the work. “The early gains really do persist,” Eisele says. “There is never this big leap in mortality.”
One reason may be that nets never provide “perfect protection,” says Günther Fink, an epidemiologist at the Swiss Tropical and Public Health Institute who helped coordinate the follow-up study. Instead, they reduce the exposure of children to mosquito-borne disease during their most vulnerable years. Later, when children are older and less likely to be using nets, “the [infected] bites arrive,” Fink says. But by then the children are stronger and can build up some immunity without a high risk of severe disease or death.
Eisele notes that programs to distribute bed nets were accompanied by other malaria control measures, including rapid tests and effective drugs that also help lower the overall number of malaria infections, leaving children less vulnerable to any possible rebound effect.
The results are also reassuring for those deciding how to use the newly approved malaria vaccine, which offers incomplete protection that seems to fade over time—prompting fears it might not have a long-term benefit. The new study demonstrates that even modest investments and imperfect tools “have a long-lasting impact in individuals’ lives,” Alonso says. Protecting the most vulnerable, small children in high-risk areas, “really pays off long term.”