sentences:  Between September 1951 and July 1952 nearly 55,000 children were involved in a clinical trial of the anti-polio serum.The results of the trial were promising; the serum was shown to be about 80% effective in preventing the development of paralytic poliomyelitis, and protection was shown to last for 5 weeks if given under tightly controlled circumstances.The serum was also shown to reduce the severity of the disease in patients who developed polio.