OBJECTIVES: To evaluate changes in all-cause pneumonia admissions following the introduction of childhood PCV immunisation in Hong Kong, using interrupted time series (ITS) analysis.

METHODS: In this population-based quasi-experimental study, we used Clinical Data Analysis and Reporting System (CDARS), which contains the electronic medical records of all public hospitals in Hong Kong, to evaluate the vaccine effect. We identified target patients with a principal diagnosis of pneumonia between 2004 and 2017 and applied the segmented quasi-Poisson regression model to estimate the change of all-cause pneumonia admission after the childhood PCV introduction, adjusting for the six-month time lag of vaccine effect, seasonality, and autocorrelation. Subgroups analysis was conducted for children and adolescents (0-19 years), adults (20-64 years) and older adults (≥65 years).

RESULTS: From 2004 to 2017, the monthly age-standardised incidence of all-cause pneumonia admission fluctuated between 2.83 and 7.47 per 10,000 persons. There was a non-significant decrease in the trend of all-cause pneumonia admission after PCV7 introduction (incidence rate ratio: 0.999, 95% CI: 0.998, 1.001). We also observed a non-significant trend change in the subgroup analysis.

CONCLUSIONS: After the introduction of childhood PCV immunisation, there is no significant reduction in all-cause pneumonia admission among the overall population in Hong Kong. The herd immunity of childhood PCV immunisation cannot be determined in adults. Future study should consider the confounding effect from influenza pandemic and climate changes on this observation.