"During the early efforts to develop a vaccine against polio, researchers discovered that the virus could cause disease not only in humans, but also in monkeys. This led to early field trials in the 1930s of vaccine candidates developed using material taken from polio-infected monkeys, such as monkey spinal cords. These candidates proved dangerous, sometimes causing paralysis in the limb where the vaccine was administered. Vaccines derived using nervous system tissue have a higher side effect profile than those developed using other methods (the myelin in the vaccine material can stimulate an adverse neurological reaction). The trials ceased, and researchers moved on to find another way to grow the virus for vaccine development."