Developing a vaccine against a parasite is inherently more difficult than developing a vaccine against a virus. A virus exists in one form and has a simple replication cycle; a parasite develops in various stages, all of which can exist in the body at once. The PfSPZ vaccine solves this problem by eliminating the malaria parasite at the beginning of its life cycle. When injected by the mosquito, sporozoites travel through the blood stream to the liver, where they start replicating. The new vaccine produces a T-cell response that works within the liver, essentially killing the parasite in its nursery. “The time in the cycle,” Fauci told me, “is critical to prevent infection.”
The study at NIH was small. Before the end of this year, there will be larger trials in Tanzania and Mali, supported by the Tanzanian government, a Swiss institute and the NIH. Malariologists will test the durability of the vaccine’s protection and its effectiveness against variations of the parasite in the wild (there are many strains of plasmodium falciparum that cause malaria in humans).
If the vaccine works as promised, it would be an extraordinary scientific milestone: the first highly effective vaccine against a parasite. And this particular parasite has been living in human hosts, and killing them, since humans evolved. Hundreds of thousands of people die of malaria each year, mainly children under 5. A vaccine that prevents infection — rather than treatments that modify the severity of the disease — is important to malaria eradication. Even people with a low level of parasites and no symptoms can transmit the disease, through mosquitoes, to others.
There are serious obstacles in moving this type of vaccination to the necessary scale (beyond, say, to tourists and soldiers). The marvel of the production process — involving individual mosquito surgeries — is itself a constraint. It would need to be somehow automated. It is a problem that the vaccine is delivered intravenously; relatively untrained health workers can’t be expected to find and hit the veins of children, which any medical intern will tell you are maddeningly small. A better delivery technology is required. A five-dose regimen is difficult to implement. And as with other temperature-sensitive vaccines, a continuous cold chain is essential.
Yet if public health officials were content to whine about obstacles, mass treatment for AIDS never would have been undertaken. On a malaria vaccine, Fauci sees a number of problems “we haven’t cracked yet.” But, he continues, “Once you have proven the concept, everything else is engineering. The stakes are so high. A baby dies every 60 seconds from malaria. I can’t imagine that some engineering genius can’t figure these things out. Let’s go for it.”