OBJECTIVES : Nine years after its introduction in the US, PCV was introduced to Hong Kong (HK) childhood immunisation program in September 2009; however, the burden of pneumonia remains high in the post-PCV period. It is important to investigate the herd immunity of childhood PCV immunisation on pneumonia hospitalization to inform further vaccination policies. This study aimed to evaluate changes in all-cause pneumonia admissions following the introduction of childhood PCV immunisation in HK, using interrupted time series (ITS) analysis.

METHODS : In this population-based study quasi-experimental study, we used Clinical Data Analysis and Reporting System (CDARS), which contains the electronic medical records of all public hospitals in HK, to evaluate the vaccine effect. We identified target patients with a principal diagnosis of all-cause pneumonia between 2004 and 2017 and applied segmented quasi-Poisson regression model to estimate the change of pneumonia admission after the childhood PCV introduction. Whereas there was an H1N1 pandemic during 2009-2011 in HK, we decided to take January 2012 as the intervention time point to minimize the potential bias associated with the influenza outbreak. Subgroup analysis was conducted by dividing age into seven groups.

RESULTS : From 2004 to 2017, the monthly age-standardised incidence of pneumonia admission fluctuated between 2.83 and 7.47 per 10,000 persons. There was a marginal decrease in the trend of all-cause pneumonia admission after PCV13 introduction (incidence rate ratio: 0.997, 95% CI: 0.995, 0.999). We also observed a similar decreasing trend in older adults aged 65 years or above but not the younger age groups.

CONCLUSIONS : After the introduction of childhood PCV13 immunization, there is a significant reduction in pneumonia admission among the overall population in HK. Moderate herd immunity of childhood PCV13 immunisation was found in older adults. Future study should investigate the long-term effect of PCV immunization under the circumstance of possible serotype replacement upon the vaccine introduction.