An experimental malaria vaccine called PfSPZ has shown when it was found to have blocked the disease in early clinical trials, according to a new study published by Science magazine on Thursday. Researchers are calling the vaccine a breakthrough while also cautioning that PfSPZ isn’t ready for prime time just yet. The vaccine, which is made using a weakened form of the disease, was administered in varying doses to a group of more than three dozen volunteers. Six people, each of whom were given a full five doses of the vaccine, were unable to contract malaria when exposed to the disease, the study says. This is the first time any vaccine has achieved 100% effectiveness in any trial, researchers report.