In July last year, GlaxoSmithKline ($GSK) submitted its malaria vaccine RTS,S for regulatory review by the European Medicines Agency. Because there are no existing malaria vaccines, Glaxo says that a vaccine to be used "alongside other measures such as bed nets and anti-malarial medicines" would be an advance in malaria control. However, a new study published Monday might have GSK rethinking the bed nets.
The study results, published online in Proceedings of the National Academy of Sciences, showed that in some cases, the combination of bed nets and a vaccine can increase the number of malaria cases.
According to a release, there are more than 20 malaria vaccine candidates in development, but none are approved. To get around this, a team led by the University of Michigan used a mathematical model of malaria transmission to look into what happens when vaccines and bed nets are used together.
"The joint use of bed nets and vaccines will not always lead to consistent increases in the efficacy of malaria control. In some cases, the use of vaccines and bed nets may actually make the situation worse," said Mercedes Pascual, a professor in the university's Department of Ecology and Evolutionary Biology, who co-authored the paper.
There are three different types of malaria vaccines in development: preerythrocytic vaccines (PEVs), which reduce the chance of infection after being bitten by a malaria-carrying mosquito; blood-stage vaccines (BSVs), which reduce the severity of disease; and transmission-blocking vaccines (TBVs), which prevent mosquitoes from transmitting malaria to other people after biting vaccinated individuals.
Somewhat counterintuitively, TBVs, which don't directly protect the vaccinated individual from malaria infection, work best with bed nets. This is because malaria immunity is "complex" and "transient." The Malaria Vaccine Initiative and Switzerland-based Mymetics are working on malaria vaccines, one of which is a TBV.
If a patient survives a first malaria infection, he or she will develop a partial immunity, which will reduce the risk of severe malaria illness in the future, said the release. This immunity will wane after a year or two, but subsequent bites will help the individual retain that immunity. However, this natural immunity can be compromised by the combination of bed nets and certain vaccines.
"This complexity is at the heart of why it has been so hard to develop any sort of malaria vaccine," said co-author Andrew Dobson of Princeton University.