The Epidemiology of Pertussis and Timeliness of Pertussis immunisation in New Zealand

Child health Immunisation Disease management New Zealand

Introduction: Despite increasing immunisation coverage in recent years, pertussis remains a leading cause of immunisation preventable disease in New Zealand. The current pertussis epidemic in New Zealand began in August 2011 and did not show signs of abating until May 2013. The current New Zealand immunisation schedule recommends that the first three doses of pertussis vaccine be received at six weeks, three months and five months of age. These are known as the primary series. A delay in receiving any dose of the primary series delays the time to developing effective immunity. It is not known how many children in New Zealand have delays between doses of pertussis vaccine, nor the extent of the delays. The aims of this study were as follows:1To provide an overview of the history of the epidemiology of pertussis as well as recent pertussis epidemiology in New Zealand.2To describe the coverage and timeliness of the primary series of pertussis immunisations for New Zealand and separately for children living in the greater Wellington region.3To detect differences in timeliness of immunisations by calendar year, socio-demographic characteristics, and by region for New Zealand and separately for children living in the greater Wellington region.An immunisation is considered not timely if it was not received within four weeks of the age specified by the New Zealand immunisation schedule. The definition of timeliness according to age works as long as the first vaccine dose is received at six weeks of age. However, if the first dose is received when the infant is older than six weeks, the timing of the second and third dose should be adjusted to maximise antibody production. Therefore, a second definition of timeliness was explored. Specifically, due dates for the second and third doses were calculated according to the recommended interval between doses. Between doses one and two the interval is scheduled to be six weeks. Between doses two and three the interval is scheduled to be two months. MethodsThe study population included all children born in New Zealand between 2007 and 2012 as recorded on the National Immunisation Register (NIR). Children excluded from the study were those: whose date of death was recorded as being before their date of birth, whose gender was recorded as 'unknown' or 'indeterminate', who were not included in the NIR (i.e opted off), who were not recorded as having received at least one pertussis vaccine dose, who had records of more than four pertussis vaccine doses, who had records of two pertussis vaccine doses within four weeks of each other, who were recorded as having died before receiving their first pertussis vaccine dose and who were recorded as receiving a pertussis vaccine dose before their date of birth. The study is a retrospective cohort study. The population was followed retrospectively over time to observe the occurrence of pertussis immunisation. Descriptive observations as well as multivariate regression analyses using 'R' were conducted. Results371,587 children recorded in the National Immunisation Register were analysed for pertussis immunisation occurrence and timeliness. Timeliness of dose one was 89.0% for the whole New Zealand population. Timeliness of dose two was 87.8% and dose three was 83.1%. Overall 71.2% of the population received all three doses on time. Timeliness varied within each of the population subgroups of ethnicity, socio-economic deprivation, calendar year of vaccination and regions within New Zealand. Children of Asian ethnicity had the highest percentage of children receive all their immunisations on time (83.4%), followed by European ethnicity (76.3%). Māori had the lowest percentage with on time vaccination across all doses and had 52.5% of children receive all their immunisations on time. Timeliness was significantly different across all ethnicities (p

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