Experiences with polio vaccination and herd immunity in Finland.

Since the mass vaccination in 1960, infants have been vaccinated systematically with inactivated polio vaccine. By school entry 97% of children have received complete primary vaccination. Since 1964 no case of poliomyelitis has been found in Finland. An intensified search for polioviruses among patients with polio-like diseases, among preschool children and in sewage did not reveal any polioviruses, giving evidence of their disappearance. Studies on immunity showed unexpectedly large numbers of preschool children without detectable antibodies to type 1 and especially to type 3. Their response to a booster dose revealed a secondary type reaction, indicating immunity. Thus, antibodies in low or even undetectable titres protect individuals from polio infection and furthermore may eliminate polio viruses from the country.