In 2011, a clinical trial of PfSPZ given under the skin reported disappointing results, protecting only two of 80 subjects. But the need to deliver the vaccine intravenously "is not a show-stopper", says Hoffman, noting that the volume of vaccine — 0.5 milliliters — is tiny and requires a tiny syringe, although the company is exploring ways to improve the intravenous delivery system.
Another logistical hurdle, says Hill, is that the vaccine must be kept frozen in liquid nitrogen vapour phase. Hoffman argues, however, that the vaccine can piggyback on veterinary infrastructure in places that use liquid nitrogen to store and transport veterinary vaccines and semen for artificial insemination of livestock. "If you can carry semen into the deep Saharan belt and remote areas, why can't you do that for a human vaccine?" says Marcel Tanner, director of the Swiss Tropical and Public Health Institute in Basel, Switzerland, which is a sponsor of the trial in Tanzania.
"Which of the logistical challenges can be managed and which will become show-stoppers can be difficult to predict," says David Kaslow, director of the PATH Malaria Vaccine Initiative in Washington, DC, a public–private partnership for malaria-vaccine development.
Kappe hopes the trial results will encourage funders to invest more in optimizing this vaccine approach. "If we were talking about an HIV vaccine, there would be no question about investing in this type of success," he says.