Abstract
Background
While the National Immunisation Schedule includes pertussis-containing vaccines for infants in the first six months, and overall immunisation coverage is over 90% at age 12 months, infants in Aotearoa experience higher rates of hospitalisation for pertussis compared to older peers.[1] Pertussis has epidemic outbreak cycles every two to five years with a recent outbreak commencing in 2017.[2] Community immunity interventions (including maternal vaccination) are recommended to disrupt pertussis transmission to infants.[3] Since 2015 all pregnant women have been eligible for pertussis-containing vaccines.
The aim of this research was to describe and compare the hospitalisation rates of infants aged under 12 months for pertussis in Aotearoa pre- and post-implementation of pertussis vaccination for pregnant women.
Methods
A retrospective analysis of acute and arranged hospital discharges coded as pertussis amongst children aged under 12 months of age from 2000 to 2017.
Results
The hospitalisation rate of infants under 12 months between 2000 and 2017 was 190 per 100,000 live births). Hospitalisation rates were highest for infants aged 6 weeks to under 3 months, and relative risk was almost double that of infants aged under 6 weeks. Hospitalisation rates were highest for infants aged under 12 months if they resided in areas of high deprivation (relative risk nearly four times that of infants living in areas of lowest deprivation), or infants were of Pacific or Māori ethnicities (relative risk was double that of European/Other infants).
In the five years following introduction of maternal immunisation, pertussis hospitalisation rates for infants aged 6 weeks to under 3 months showed a reduction in risk compared to the other age groups.
Conclusion
Internationally the extension of pertussis vaccination to pregnant women appears to have an age-dependent protective effect for infants aged under 6 months. In New Zealand, the observed decrease for each age group in rates of pertussis hospitalisations post introduction of maternal vaccination was greater than those observed following earlier epidemic peaks. Further work is required to ascertain the immunisation status of the mother and of the hospitalised infant, to better understand the relationship between maternal and infant immunity against pertussis.
References
1. Duncanson, M., et al., Health and wellbeing of under-five year olds in New Zealand 2017, in Health and wellbeing of under-five year olds. 2019, New Zealand Child and Youth Epidemiology Service: Dunedin, New Zealand.
2. Ministry of Health. National outbreak of whooping cough declared. 2017 [cited 2019 21 May]; Available from: https://www.health.govt.nz/news-media/media-releases/national-outbreak-whooping-cough-declared.
3. World Health Organisation, Pertussis vaccines: WHO position paper – August 2015. 2015.