The US government licensed the Salk vaccine in 1955, so that 1956 was the first year with 12 months of data on vaccine impact.

Rising living standards in the decade after World War II had led to later and more frequently symptomatic infection with polio, so that by 1954 the polio epidemics were regular front page news. The decade before 1955 saw polio in the US ranging from 2000 to 3500 cases per year.

Nathanson and colleagues review the evidence on decline in incidence and shift in age distribution seen in 1956. The dramatic decline in polio cases did not alter the view that transmission continued. The authors comment:

¨the primary effect of vaccine appears to be the prevention of invasion of the central nervous system and thereby the prevention of paralysis. This limitation on the effectiveness of the vaccine may be associated with the evidence that poliovirus did spread rather extensively in various populations during 1956, not only in Chicago, but in Louisiana, Utah, Idaho, California, and elsewhere. In spite of relatively widespread, but incomplete, vaccination, these populations experienced high incidence of disease, particularly among preschool children in all socioeconomic groups.

¨The immediate public health implication of the experience in 1956 is that substantially higher levels of immunity must be achieved among all elements of the population.¨

Full text at http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=2031288&blobtype=pdf

Though the 1956 authors do not say so, the publicity surrounding the Cutter incident of 1955 may have slowed acceptance of IPV. For a retrospective look at the Cutter incident and its impact on product liability law, see ¨The Cutter Incident, 50 Years Later,¨ New England Journal of Medicine, http://content.nejm.org/cgi/content/full/352/14/1411