Some bed net-vaccine combinations may worsen malaria

Joint use of bed nets and vaccines will not always lead to consistent increases in the efficacy of malaria control, and sometimes their use may actually make the situation worse, a new study suggests.
"The joint use of bed nets and vaccines will not always lead to consistent increases in the efficacy of malaria control," said Mercedes Pascual, a professor in the University of Michigan Department of Ecology and Evolutionary Biology.
"In some cases, the use of vaccines and bed nets may actually make the situation worse," said Pascual.
"Specifically, our study suggests that the combined use of some malaria vaccines with bed nets can lead to increased morbidity and mortality in older age classes," said Pascual.

The malaria vaccines under development fall into three categories, each focusing on a different stage of the malaria life cycle. That cycle involves human hosts and Anopheles mosquitoes infected with Plasmodium parasites.
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.
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.
"Ironically, the vaccines that work best with bed nets are the ones that do not protect the vaccinated host - the bed net does that - but instead block transmission of malaria in mosquitoes that have found an opportunity to bite vaccinated hosts," Artzy-Randrup said.

In contrast, both BSV and PEV malaria vaccines, when combined with bed nets, actually increased the number of malaria cases seen in the modelling study.
Interactions between the bed nets and those vaccines reduced levels of natural immunity in the population, increasing morbidity in older age classes.
The research was published in the journal PNAS.