Polio Seroprevalence in Thailand: Assessment of Outbreak Risk by Age Cohorts

Abstract

We conducted a serological survey to evaluate the population’s antibody level against three types of polio virus and identify high risk groups. We analyzed stored serum samples from a hepatitis immunity study conducted in 2004 on people born between 1928 and 2004. These samples were categorized into nine age cohorts and selected by random sampling. Antibody titers were tested by micro-neutralization. A protective level was defined as greater than 1:8. Protective antibody level against poliovirus and geometric mean titer (log2 reciprocal) were described by types of polio virus in the vaccine and birth cohorts. A total of 1,712 samples were tested. Protective antibody level against poliovirus type 1 was 90.9% while that of type 2 was 94.7% and type 3 was 83.9%. Means titers were 6.0 for type 1, 6.7 for type 2 and 4.9 for type 3. In the different birth cohorts, the antibody levels were the lowest against poliovirus type 2 (89.9%) in those who were born during 1955-1964. For poliovirus types 1 and 3, percentages in the 1975-1984 birth cohorts were less than 80%. Protective antibody level against the three types of poliovirus among the population in Thailand was assumed to be sufficient to generate herd immunity. People born during 1975-1984 were at risk and should be targeted for immunization if a polio outbreak occurred.