Preliminary report on vaccination in Croatia against poliomyelitis with type 1 (CHAT) and type 3 (W-Fox) attenuated polioviruses of Koprowski.

The incidence of poliomyelitis in the People's Republic of Croatia has progressively increased since 1945, and in 1953 and 1960 there were serious epidemics. In order to protect the age-groups at greatest risk a mass vaccination campaign was carried out with Koprowski live-virus vaccine in the early spring of 1961, covering the entire population aged 3 months to 20 years. Altogether 1 339 244 persons were given type 1 (CHAT strain), and 1 287 909 received type 3 (W-Fox). Over 100 000 persons were estimated to possess no pre-existing antibody to any of the three types of poliovirus.Although no unvaccinated control group was set up, it is considered that the epidemiological and serological data indicate the safety and efficacy of the vaccination. The serological conversion rate was 91.5% for type 1 and 93.5% for type 3. No post-vaccinal reactions were observed. The usual summer peak in incidence did not occur in 1961, altogether 6 cases being recorded from June (when immunity is considered to have been established) to December. This is the lowest figure since 1945.