Background: Australia introduced a fifth dose of pertussis vaccine at 4 to 5 years of age in 1994, the first country to do so for some 40 years. We report trends in national pertussis notifications from 1993 to 2001.
Methods: Notified pertussis cases were analyzed by age and year of disease onset.
Results: Before the fifth dose was introduced, notification rates were higher among 5- to 9-year-olds than 10- to 14-year-olds (76 per 100 000 population vs.65 per 100 000). As more 5- to 9-year-olds became eligible for the fifth dose, their notification rates as a group and by year of age progressively fell to below those of 10- to 14-year-olds, consistent with a vaccine effect. Comparison of notification rates for the epidemic years of 1997 and 2001 shows that 5- to 10-year-olds (eligible for fifth dose) had lower notification rates in 2001 (61 per 100 000) than 5- to 10-year-olds in 1997 (196 per 100 000). This contrasts with children who were not eligible for the fifth dose (12- to 14-year-olds), who had higher notification rates in 2001 (223 per 100 000) than 12- to 14-year-olds in 1997 (160 per 100 000).
Conclusions: The pattern of age-specific notification rates provides strong evidence that the fifth dose reduced the incidence of pertussis in older children. It will be important to track the impact of the fifth dose on adolescent pertussis notifications to assess the duration of vaccine-acquired immunity.