In a new study, scientists have found that combining insecticide-treated bed nets with vaccines may actually worsen some cases of malaria rather than helping them.
A University of Michigan-led research team used a mathematical model of malaria transmission to find out what will happen when vaccines and bed nets are used together.
Professor Mercedes Pascual, co-author of the study, said that the joint use of bed nets and vaccines would not always lead to consistent increases in the efficacy of malaria control. Their study suggested that the combined use of some malaria vaccines with bed nets can lead to increased morbidity and mortality in older age classes.
Preerythrocytic vaccines, or PEVs, aim to reduce the chances that a person will be infected when bitten by a disease-carrying mosquito. Blood-stage vaccines, or BSVs, don't block infection but try to reduce the level of disease severity and the number of fatalities.
Transmission-blocking vaccines, or TBVs, don't protect vaccinated individuals against infection or illness. But they prevent mosquitoes from spreading the disease to others after biting a vaccinated person.
Because TBV vaccines provide no protection to the vaccinated person but potentially reduce the rate at which others are infected, they are commonly called altruistic vaccines.
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Pascual and her colleagues found that the combination of treated bed nets and TBV vaccines achieves the most efficient control, resulting in fewer cases of malaria while increasing the probability of eliminating the disease.
In contrast, both BSV and PEV malaria vaccines, when combined with bed nets, actually increased the number of malaria cases seen in the modeling study. Interactions between the bed nets and those vaccines reduced levels of natural immunity in the population, increasing morbidity in older age classes.
Unraveling the interactions between bed nets and vaccines is especially challenging due to the complex and transient nature of malaria immunity.
A child's first malaria infection can result in severe, sometimes fatal, illness. If the child survives, he or she will gain partial immunity that reduces the risk of severe illness in the future.
Additional bites from infected mosquitoes can help the child retain that immunity, which would otherwise wane after one to two years. But the combination of bed nets and certain vaccines can undermine that natural immunity.
In 2013, there were an estimated 198 million malaria cases worldwide, including 584,000 deaths, according to the World Health Organization. Most deaths occur among children living in Africa, where a child dies every minute from malaria, according to WHO.
Between 2000 and 2013, access to insecticide-treated bed nets increased substantially. In 2013, almost half of all people at risk of malaria in sub-Saharan Africa had access to an insecticide-treated net, a marked increase from just 3 percent in 2004, according to WHO.
The paper scheduled for online publication Jan.