Artificial herd immunity to poliomyelitis in a semirural community in South Africa

The sera of 139 children aged between 8 months and 6 years from the town of Port Alfred in the eastern Cape Province were examined for antibodies to the three types of poliovirus. A protective level of antibody to type 1 virus was found in 73.4% of the sera. The figures for types 2 and 3 were 94.2% and 68.3% respectively. The results indicate good herd immunity to poliomyelitis. No cases of paralysis or isolations of poliovirus had been reported from the area for the 6 years preceding the survey. The high degree of immunity is therefore unlikely to be due to circulating 'wild' virus to any great extent. It must be ascribed mainly to the immunity achieved by the administration of an average of 2.8 doses per child of trivalent live poliovirus vaccine. The low figure of 47.4% for immunity to all three types of virus could be due to enterovirus interference and inhibitory factors. A fourth dose of vaccine was thought to be indicated and is now given as a routine measure. Further investigations on the response to this will be undertaken.