AGE-DIFFERENCES IN IMMUNITY AGAINST WILD AND VACCINE STRAINS OF POLIOVIRUS PRIOR TO THE 1988 OUTBREAK IN ISRAEL AND RESPONSE TO BOOSTER IMMUNIZATION

During the 1988 type 1 polio outbreak in Israel, most cases occurred in previously vaccinated subjects aged 11-30 years, suggesting a possible age-related immunity deficit against the wild virus responsible for the outbreak. We examined type 1 poliovirus neutralizing antibody titres against the Sabin strain, the standard wild strain (Mahoney), the wild strain responsible for the 1988 outbreak and a previous wild strain from the region, on frozen sera drawn prior to the mass vaccination campaign from subjects aged 6 to 40 years. Response to vaccination with oral poliovaccine (OPV) was examined in a subgroup aged 18-40 years. At all ages, the highest antibody titres prior to the outbreak were against the Sabin strain. Geometric mean titres (GMTs) against both the Sabin strain and the wild Mahoney strain were significantly higher in the age groups 6-7, 12-13 and 30-40 years compared with the 18-29-year-olds. For the other wild strains, the GMT for those aged 30-40 years was significantly and substantially higher than in the other age groups, followed by the 12-13- and 6-7-year-olds and lowest in those aged 18-29 years. Following vaccination with OPV in subgroups aged 18-29 and 30-40 years, GMTs against Sabin and all wild strains were similar to each other and in both age groups. These findings suggest that there was a relative immunity gap against the wild type 1 strains in the age group that lacked prior exposure to wild virus and had received the last OPV dose more than 17 years previously. Since a high inoculum of wild virus or intratypic antigenic changes of the poliovirus may produce disease in susceptible people, booster doses of vaccine for adults may be necessary, especially in areas where imported polioviruses are likely to occur.