Paralytic poliomyelitis in Norway since the introduction of trivalent oral vaccine: an epidemiological and virological study.

Inactivated poliovirus vaccine was replaced by oral vaccine in Norway in the autumn of 1965. This was followed by a further decrease in the number of cases of paralytic poliomyelitis. The present study was undertaken to investigate whether the few and sporadic cases observed were caused by naturally occurring (wild) poliovirus or by vaccine virus. Trivalent oral vaccine from the same manufacturer was used throughout the campaign, whereas other similar studies have mainly been concerned with monovalent oral vaccine. Altogether 6 vaccine-associated cases were observed, giving a higher rate of paralytic cases per million doses of trivalent vaccine than reported by most authors. Epidemiological and virological findings suggest that wild poliovirus has not been circulating in the Norwegian population during recent years.