Influenza vaccine effectiveness in hospitalised Hong Kong children: Feasibility of estimates from routine surveillance data.

Background: Hong Kong has a high burden of influenza hospitalisation. This study estimated influenza vaccine effectiveness in hospitalised Hong Kong children aged 6 months to below 6 years using data potentially obtainable from routine surveillance sources.

Methods: This 'test-negative' case-control study was conducted over two summer and one winter influenza seasons in five public Hong Kong hospitals during 2015 and 2016. Patients admitted for febrile and/or respiratory-associated illnesses who met inclusion criteria were invited to participate. Case-patients were respiratory-associated admissions with nasopharyngeal aspirate or nasopharyngeal swab specimens obtained during the first 48 h of hospitalisation that tested positive for influenza A or B, whereas control-patients were those with specimens that tested negative for both influenza A and B. Reliability of a routinely collected influenza immunisation status form was evaluated. Vaccine effectiveness for administration of full or partial series of influenza vaccination was calculated as 1 minus the odds ratio for influenza vaccination history for case-patients versus control-patients.

Results: 2900 eligible subjects had influenza vaccination status available. A simple record form, designed to collect upon admission information on influenza vaccination status, was found to be reliable when compared to confirmed vaccination status from immunisation records and guardians' self-reports. Influenza vaccine effectiveness for preventing influenza A or B hospitalisation in children aged from 6 months to below 6 years during the period June 2015 to November 2016 was 68% (95% confidence interval [CI]: 55%, 77%) from unconditional analyses and 64% (95% CI: 46%, 75%) from conditional analyses.

Conclusions: Seasonal influenza vaccine was effective in preventing hospitalisation from influenza A or B in young Hong Kong children during 2015 and 2016. As influenza vaccination status is not currently routinely recorded, implementation of an influenza immunisation status form in all paediatric wards, and centralising the data in Hong Kong's central computerised database, could provide real-time monitoring of influenza vaccine effectiveness.