The seroepidemiology and transmission dynamics of varicella in Australia

To enhance our understanding of the epidemiology and transmission dynamics of varicella in the pre-vaccine era we performed a serosurvey using opportunistically collected sera submitted to diagnostic laboratories across Australia during 1997-1999. A representative sample by state and sex of 2027 sera from persons aged 1-49 years was tested using an enzyme immunoassay method. The average age of infection and age-specific forces of infection (the probability that a susceptible individual acquires infection) were calculated using published methodologies. Seropositivity increased with age, with 83% of sera positive by ages 10-14 years. The highest force of infection was in the 5-9 years age group (0.195 per susceptible year) followed by the 0-4 years age group (0.139 per susceptible year) and the average age of infection was 8.15 years. These results provide valuable baseline information to measure the impact of vaccination and indicate that vaccination should be aimed at children less than 5 years of age, although further modelling using the serosurvey data is warranted.