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.
The marked drop in reported polio cases from 1955 to 1956 might provide final proof of the value of the vaccine if the number of polio cases in each of the previous years had been relatively constant. As seen in the first diagram, this is not the case. There have been wide swings in the number of polio cases from year to year. Beginning in the 1930s, when reports of polio became fairly reliable, there were a number of years—particularly in the late thirties when there were many fewer cases than in 1956. Following this period, there was a rise in the early 1940s, particularly in 1944 when 19,029 cases were reported. In 1947, for no apparent reason, there was a sharp drop to about 10,000 cases. After that, there were a number of "high polio years" reaching a peak in 1952 with 57,879 cases, which was followed by a drop-off to about half that number in 1955. These fluctuations in the number of cases per year have no known explanation and occur not only in the United States but in many parts of the world. It is of interest that a sharp drop also occurred in England and Wales in these same two years, 1955 and 1956, even though in those countries only 200,000 children had received but one or two injections in a program which began in the late spring of 1956. It is, therefore, impossible to tell whether the decrease from 1955 to 1956 in the United States is a result of the polio vaccine program or whether it is just another sharp swing in the usual pattern of the disease.
The total number of cases of polio reported each year includes both paralytic and nonparalytic forms of the disease. When polio occurs without paralysis, it may be difficult to diagnose, particularly in the absence of an epidemic. Nonparalytic polio has to be differentiated from infections due to other viruses, a distinction which medical advances have made possible only during the past few years. When such other virus infections are recognized in epidemic form, as occurred in Iowa in 1956, these cases are properly not included in the total annual figure for polio. Improvement in diagnosis has tended to decrease the number of reported cases of nonparalytic polio in recent years. This in turn makes comparisons of total cases in recent years with previous years less reliable.
Paralytic cases, however, are easily recognized, and paralysis only rarely occurs in other infectious diseases. Thus the total number of paralyzed cases is more reliable for year-to-year comparison. When the paralytic cases to December for 1955 and 1956 are compared, the decrease is much smaller than that of the total cases. There was a drop from 10,405 paralytic cases in 1955 to 6565 in 1956, a decrease of about one third, although there is a possible 5 to 10 per cent error here because of incomplete reports. Reliable records on numbers of paralytic cases for the United States are available for only the last two or three years, and they are, therefore, not precisely helpful at this time in interpreting the sharp decrease of this year. However, the seriousness of the paralytic disease and the reliability of reports on such cases will make this the best index for measuring the effect of polio vaccine in the future.
We are encouraged that in 1956 there was only one epidemic in a major American city, Chicago. This Chicago epidemic was accompanied by an increase in polio in surrounding Cook County and northern Illinois, but this, as well as the other state epidemic in 1956, in Louisiana, was definitely smaller than the most serious epidemics of previous years. It is quite possible that polio vaccination completed before the beginning of the 1956 polio season had something to do with the smaller size. of these epidemics. In future years, if the most serious epidemics continue to decrease in size, this will also be an important index of the value of the vaccine. On the other hand, the intense polio vaccination campaign begun after the start of the epidemic in Chicago had no effect on the course of that epidemic. As can be seen in the second diagram, the upswings and downswings on the epidemic curve are of the same shape, and balance each other as is usual in this disease. If the vaccine given after the start of the epidemic had had a real effect on it, there ought to have been a sharp drop in the number of cases, and the downswing of the curve in the diagram would have been a straight line. One can not expect that polio vaccine given after the start of an epidemic would make any real difference in that particular outbreak of disease, because once an epidemic has started, there is not sufficient time for the vaccine to develop enough protective antibodies in those exposed to the disease.
Complicated comparisons of total and paralytic cases and of epidemics of polio from year to year would be completely unnecessary if it could be shown that children who had received complete vaccination—three polio vaccine injections at the recommended time intervals—would not be seriously attacked by this disease. Unfortunately, this is not so. In 1956 there were over one hundred vaccine failures reported from various parts of the United States. Some of these patients have had severe paralysis and at least two have died. Since there is no way of counting the number of children who had complete vaccinations by the beginning of the 1956 polio season, it is impossible to tell how important this group of cases really is in evaluating the effect of the vaccine.