Age-specific effectiveness following each dose of acellular pertussis vaccine among infants and children in New Zealand.

BACKGROUND: Though it is believed the switch from whole cell to acellular pertussis vaccine has contributed to the resurgence of pertussis disease, few studies have evaluated vaccine effectiveness (VE) and duration of protection provided by an acellular vaccine schedule including three primary doses but no toddler-age dose. We assessed this schedule in New Zealand (NZ), a setting with historically high rates of pertussis disease, and low but recently improved immunisation coverage. We further evaluated protection following the preschool-age booster dose.

METHODS: We performed a nested case-control study using national-level healthcare data. Hospitalised and non-hospitalised pertussis was detected among children 6weeks to 7years of age between January 2006 and December 2013. The NZ National Immunisation Register provided vaccination status for cases and controls. Conditional logistic regression was used to calculate dose-specific VE with duration of immunity examined by stratifying VE into ages aligned with the immunisation schedule.

RESULTS: VE against pertussis hospitalisation was 93% (95% confidence interval [CI]: 87, 96) following three doses among infants aged 5-11months who received three compared to zero doses. This protection was sustained through children's fourth birthdays (VE⩾91%). VE against non-hospitalised pertussis was also sustained after three doses, from 86% (95% CI: 80, 90) among 5-11month olds to 84% (95% CI: 80, 88) among 3-year-olds. Following the first booster dose at 4years of age, the protective VE of 93% (95% CI: 90, 95) among 4-year-olds continued through 7years of age (VE⩾91%).

CONCLUSIONS: We found a high level of protection with no reduction in VE following both the primary course and the first booster dose. These findings support a 3-dose primary course of acellular vaccine with no booster dose until 4years of age.