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.