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Healthy adults immunized with an experimental malaria vaccine may be completely protected from infection, according to government researchers.

The vaccine, called PfSPZ, is being developed by an American biotech company Sanaria and contains weakened forms of the live parasite — Plasmodium falciparum — responsible for causing malaria.

The vaccine is made from sporozoites, or early-stage parasites extracted from infected mosquitoes, which are the most common carriers of P. falciparum. The sporozoites were incapacitated so they can’t develop into disease-causing maturity, and infused intravenously into vaccinees. Among 40 healthy volunteers, those who received the higher doses of the vaccine showed more antibodies against the malaria parasite’s proteins than those getting lower doses. When the immunized participants where tested with exposure to P. falciparum, none of the six who received six doses of the vaccine developed malaria, while five of the six who were not vaccinated became infected.

That’s encouraging news, say the scientists, led by Dr. Robert Seder of the Vaccine Research Center at the National Institutes of Health, who were supported by the National Institute of Allergy and Infectious Diseases (NIAID), the Walter Reed Army Institute of Research and the Naval Medical Research Center.

The trial was only the first phase of clinical testing for the vaccine, but, said Dr. Anthony Fauci, director of the NIAID in a statement, it is an “important step forward” in controlling malaria, which infects about 219 million people worldwide annually and leads to 600,000 deaths. While drug treatments can protect against infection, they are most effective when used in combination with bed nets and insect repellent.

(MORE: Early Results: A First-Ever Malaria Vaccine Protects Children)

“It allows us in future studies to increase the dose and alter the schedule of the vaccine to further optimise it. The next critical questions will be whether the vaccine is durable over a long period of time and can the vaccine protect against other strains of malaria,” Seder told the BBC.

The results, which were published in the journal Science, are especially encouraging after initially promising findings from another malaria vaccine developed by Glaxo Smith Kline proved less robust than thought. In 2012, a trial involving infants in seven African counties, where malaria is endemic, showed that the vaccine, called RTS,S, was 30% effective in protecting babies aged five to 17 months from infection. At the time, public health officials debated whether that was sufficient to start vaccinating kids in countries where the disease is more rampant. TIME wrote:

At its current power, the candidate vaccine “potentially translates to tens of millions of malaria cases among children that can be averted annually,” Dr. Tsiri Agbenyega, head of the malaria research unit at the Komfo-Anokye Hospital in Ghana and chair of the RTS,S Clinical Trials Partnership Committee, told reporters during Tuesday’s briefing. “The study found that RTS,S also reduced risk of severe malaria by 47%. That’s remarkable when you consider that there has never been a successful vaccine against a human parasite, nor against malaria.”

(MORE: Drug-Resistant Malaria Is Spreading, and It Could Be a Public Health Disaster)

But an updated report in the New England Journal of Medicine this past March found that the protection from the vaccine didn’t last. Beginning eight months after vaccination, the shot’s effectiveness started to wane, and four years later, its efficacy dropped to about 17%. “It was a bit surprising to see the efficacy waned so significantly over time. In the fourth year, the vaccine did not show any protection,” follow-up study leader Ally Olotu of the Kenya Medical Research Institute (KEMRI) Wellcome Trust Research Programme in Kenya, told Reuters in March. Researchers continue to test RTS,S in final-stage trials with over 15,000 kids, and the results are expected by the end of next year, according to Reuters.

That vaccine, however, used snippets of the malaria parasite’s proteins that were fused with proteins from the hepatitis B virus in order to activate the immune system into producing antibodies against P. falciparum. Researchers hope that the weakened, live form of the the parasite contained in PfSPZ produces more lasting responses, but more research will need to be done to see if that’s the case. It’s also not clear how practical the IV delivery of this vaccine will be, since countries where malaria is common often have weak health systems and fewer medical resources.

Still, the findings hold promise that it may be possible to protect against malaria with a vaccine, and avoid hundreds of thousands of deaths.