Seroprevalence of antibodies to poliovirus in individuals living in Portugal, 2002

The last case of poliomyelitis in Portugal caused by indigenous wild poliovirus occurred in 1986 and the country was declared polio-free in 2002. High levels of immunity must be maintained to prevent the importation of wild poliovirus. In this study, we determined the immunity against poliomyelitis of the Portuguese population in order to identify possible immunity gaps. A representative sample of 1,133 individuals older than two years residing in mainland Portugal was studied. Logistical difficulties regarding quick sample transportation precluded the Portuguese islands (Madeira and the Azores) from this study. Sera were collected in 2002 from individuals attending health clinics throughout the 18 districts of Portugal. Levels of neutralizing antibodies against poliovirus types 1, 2 and 3 were determined and a titre of >= 1:8 was defined as indicative of protected immunity. Results were expressed in international units. The antibody prevalence and the geometric mean antibody concentration (GMAC) was 91.6% (GMAC: 2.96 IU/ml), 94.2% (GMAC: 5.03 IU/ml) and 75% (GMAC: 0.53 IU/ml) for poliovirus types 1, 2 and 3, respectively. For poliovirus types 1 and 2, antibody prevalence was close to or above 90% in the majority of age groups. For poliovirus type 3, antibody prevalence was below 80% in teenagers and young adults. Our study shows that the Portuguese are well protected against poliovirus types 1 and 2. For poliovirus type 3, the suboptimal antibody levels observed in teenagers and young adults suggest the need for a booster dose to minimise the risk of wild poliovirus importation.