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

Abstract

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: 1. To provide an overview of the history of the epidemiology of pertussis as well as recent pertussis epidemiology in New Zealand. 2. To 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. 3. To 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. Methods The 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. Results 371,587 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<0.001) and between European and Māori (p<0.001). Timeliness varied by deprivation level (as measured by NZDep deprivation index) for all doses with children living in the least deprived areas having a higher percentage receive their doses on time compared to those living in the most deprived areas. Children living in deprivation areas 1-3 (the lowest deprivation categories) had 79.8% of children receive all their immunisations on time, followed by deprivation areas 4-7 (75.7%) and deprivation areas 8-10 (65.8%). Timeliness was significantly different across the three NZ Deprivation categories for all doses (p<0.001). The Southern region had the highest percentage of children receive all their immunisations on time (78.9%) followed by Central (73.3%) and Northern (70.1%). Midland had the lowest percentage with on time immunisation across all doses and had 63.8% of children receive all their immunisations on time. Timeliness was significantly different between all regions and across all doses (p<0.001). There were 39,293 children in the greater Wellington region (Wairarapa, Hutt Valley and Capital and Coast District Health Boards). Timeliness of dose one was 90.3% for the Wellington region. Timeliness of dose two was 90.3% and dose three was 87.0%. 76.0% received all three doses on time. The Wellington region had a higher total percentage of children receiving dose one on time and intervals one and two on time compared to the rest of the country. Similar trends and patterns of results were seen in the greater Wellington region when compared to the whole of New Zealand. There was a significant difference in timeliness percentages between all District Health Boards (DHBs) with Hutt Valley DHB having the highest percentage of children receiving all their doses on time (76.1%) followed by Capital and Coast (76.0%) and Wairarapa (75.4%) DHBs. There was a significant difference in timeliness percentages between all DHBs (p<0.001). Discussion and conclusions This study found that only 71.2% of infants received all three pertussis doses on time leaving them vulnerable to infection themselves, and also acting as a source of infection for others. This immunisation deficiency may be contributing to pertussis remaining a poorly controlled disease in New Zealand with recurring epidemics resulting in large numbers of cases, hospitalisations and occasional deaths. On time immunisation varied across population groups and regions showing that there is work to be done to reduce these inequalities. There were significant differences between Māori and Pacific People and those living in areas of high socio-economic deprivation in particular. Policy concerning immunisation coverage and timeliness in New Zealand currently targets these populations and these results confirm that focus should remain on these groups if immunisation timeliness is to improve and rates of pertussis disease are to go down. This study shows that those who do not receive their first pertussis immunisation on time are more likely to have subsequent delays. This is an important finding which could guide interventions aimed at improving immunisation timeliness. Immunisation provides an ideal opportunity to connect with the parents or caregivers of the child, and educate them on the importance of immunisation timeliness to ensure that subsequent doses are received on time. Many primary care facilities in New Zealand have a system in place to contact parents and caregivers when their child is due for an immunisation, either by phone or mail. However, this contact can be easily forgotten when medical centres and other facilities become too busy or there are staff shortages. One solution may be to employ a dedicated nurse administrator, who is responsible for contacting parents to remind them that their child is due for an immunisation and to discuss with them the importance of timeliness. This is the first study in New Zealand to provide a comprehensive estimate of pertussis immunisation timeliness. Future research into whether the results from this study on immunisation timeliness correspond with rates of pertussis disease and hospitalisation would improve the understanding of the relationship between pertussis immunisation timeliness and disease occurrence.