 also limits the choice of variables to that which is available and of a reasonable quality. Some study limitations will therefore be related to the validity of the variables chosen for study purposes and the quality and reliability of these variables over time. The case series studies are exploratory in nature, however, it is a valuable and necessary starting point for further research. A comprehensive description of demographic, clinical and pharmaceutical variables in relation to pertussis vaccination failure has not been published before. The complimentary use of exploratory and hypothesis testing study designs is a significant strength of this study, as this design minimises potential issues with the case series studies and maximises the potential of the cohort studies by making available high-quality information based on more than just investigative hunches. This study will begin to address a major gap in paediatric pertussis prevention literature.

We are grateful to Nikki Turner for her support and feedback in the designing of this study. We also would like to thank Lottery Health Research New Zealand for the PhD in health scholarship for H.C.

Conceptualization, H.C., A.H., E.B. and H.P.-H.; methodology, H.C., A.H., E.B. and H.P.-H.; software, H.C., A.H.; data curation, A.H.; writing---original draft preparation, H.C.; writing---review and editing, H.C., A.H., E.B. and H.P.-H.; supervision, A.H., E.B. and H.P.-H.; project administration, H.C.

This research received no external funding.

The authors declare no conflict of interest.

![Study design and study objectives.](vaccines-07-00065-g001){#vaccines-07-00065-f001}

![Fully vaccinated criteria.](vaccines-07-00065-g002){#vaccines-07-00065-f002}

vaccines-07-00065-t001_Table 1

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Data sources.

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