Efficacy of Poliomyelitis Vaccine

A measure of the effectiveness of vaccination against poliomyelitis has been sought in the data from Minnesota, where the administration of Salk vaccine began in May, 1955. With the usual methods of comparison, utilizing trends in total and paralytic attack rates, the results have been difficult to interpret. The rate per 100,000 for all cases fell steadily from 131.6 in 1952 to 4.5 in 1956; but the fall began before vaccination, and the figure 4.5 is little lower than the 6.7 observed in 1947. Similarly the rate for paralytic cases fell steadily from 68.4 in 1952 to 2.1 in 1956; the fall began before vaccination, and the figure 2.1 is little lower than the minimum of 3.7 observed in 1947. The data for Minnesota are complete as regards the ratio of paralytic to total cases since 1946 and show a consistent trend downward from that year, the ratio of 46.2 for 1956 is little lower than that of 48.4 for 1954 and could be interpreted as continuing the trend. However, when a "life-table" method was applied to the data on poliomyelitis experience of vaccinated and unvaccinated groups, it indicated that two doses of the vaccine effected a significant reduction of the incidence of paralysis although single doses did not. It is concluded that Salk vaccine as utilized in Minnesota exerted a significant protective effect when two doses were given.