National serosurvey of poliovirus immunity in Australia, 1996-99

OBJECTIVE: To measure immunity to poliovirus types 1, 2 and 3 in the Australian population. METHODS: Sera were collected opportunistically from laboratories around Australia between 1996 and 1999. A representative sample by age and gender was tested for neutralising antibodies to poliovirus types 1, 2 and 3. A titre of > or = 8 was considered antibody positive and indicative of immunity. RESULTS: Of the 1,813 sera tested, 82% were antibody positive for poliovirus type 1 and 88% were positive for type 2. Immunity to type 3 poliovirus was lower overall (74%) and especially in school-aged children and young adults. For all three poliovirus types, there were more females immune than males and immunity peaked in the 2-4 years age group. The proportion of the population immune to all three types was 59%, and 3% were negative for all three types. CONCLUSIONS AND IMPLICATIONS: This is the first national serosurvey for immunity to poliovirus in Australia. Herd immunity is probably sufficient to prevent generalised outbreaks due to type 1 and 2 poliovirus, but this may not be the case for type 3. However, localised outbreaks of any poliovirus type could still occur following reintroduction unless uniformly high levels of vaccination coverage are maintained. Ongoing serosurveillance is required following the recent change back to inactivated polio vaccine.