Polio, the familiar term for the disease of the nervous system medically known as anterior poliomyelitis, is caused by a tiny living particle—a virus. Polio has its peak occurrence each summer, when parents anxiously note the location of each new case. In the past they have stood by helplessly when the disease struck nearby, watching through the passing months to learn whether this was a "polio year" in their town or state and breathing easily only when cold weather came.
Not until 1949, when Professor John Enders made his Nobel prize-winning discovery of a method of growing polio virus in large amounts in the laboratory, did the possibility of preventing the disease through the use of a vaccine become real.
When a living polio virus invades the body, depending upon the amount of the virus and the immunity of the individual, it may or may not cause polio. But it usually does stimulate the production of protective substances or antibodies. Living viruses can be changed into vaccines, which should stimulate the production of' antibodies and never cause illness. Changing a living virulent virus into a "killed" vaccine requires the addition of a substance which will destroy the sickness-producing effect without blocking the production of protective substances in the vaccinated child. This principle was applied in the development of the Salk vaccine, which is made by the addition of formalin (formaldehyde) to living polio virus. Since the Salk vaccine was first proposed, many claims have been made for it, and it has at times been the subject of public controversy. Now, after almost three years of experience, it is possible to look at the record and come to some firm conclusions about it.
In the United States in the-first forty-nine weeks of 1956, 15,128 cases of polio were reported to the Public Health Service. In 1955 over the corresponding period there were almost twice as many cases—28,816. This sharp decrease took place during the mass polio vaccination program which was begun in April, 1955. First young school children and pregnant women, and later older children and young adults, were given one to three injections of polio vaccine. At the beginning of the polio season in the early summer of 1956, it was estimated by the National Foundation for Infantile Paralysis that thirty million people, mostly children, had received one or more injections of polio vaccine. That estimate had to be based on the enormous number of bottles of vaccine distributed by vaccine manufacturers and not on the number of persons actually vaccinated. At the beginning of the program carefully planned records were kept by Health Departments of all those vaccinated under their auspices. But when the program was changed to include over-the-counter sales of vaccine to private physicians, such records no longer included all of those vaccinated.