Effectiveness of influenza vaccination on influenza-associated hospitalisations over time among children in Hong Kong: a test-negative case-control study

The protection conferred by influenza vaccination is generally thought to last less than a year, necessitating annual revaccination. However, the speed with which influenza vaccine effectiveness might decline during a year is unknown, which is of particular importance for locations with year-round influenza activity. We aimed to assess how influenza vaccine effectiveness changes by time intervals between vaccination and admission to hospital, taking advantage of almost year-round circulation of influenza in Hong Kong. In this test-negative case-control study, we analysed vaccine effectiveness in children (aged 6 months to 17 years) who were admitted to hospital in Hong Kong over 5 consecutive years (2012–17). We included those who were admitted to general wards in four public hospitals in Hong Kong with a fever (≥38°C) and any respiratory symptom, such as runny nose, cough, or sore throat. We used direct immunofluorescence assay and reverse transcription PCR to detect influenza virus infection, and recorded children's influenza immunisation history. We compared characteristics of positive cases and negative controls and examined how vaccine effectiveness changed by time between vaccination and admission to hospital with regression analyses. Between Sept 1, 2012, and Aug 31, 2017, we enrolled 15 695 children hospitalised for respiratory infections, including 2500 (15·9%) who tested positive for influenza A or B and 13 195 (84·1%) who tested negative. 159 (6·4%) influenza-positive cases and 1445 (11·0%) influenza-negative cases had been vaccinated. Most vaccinations were done by December of each influenza vaccination season. Influenza-related admissions to hospital occurred year-round, with peaks in January through March in most years and a large summer peak in 2016; pooled vaccine effectiveness for children of all ages was 79% (95% CI 42–92) for September to December, 67% (57–74) for January to April, and 43% (25–57) for May to August. Vaccine effectiveness against influenza A or B was estimated as 79% (95% CI 64–88) within 0·5–2 months of vaccination, 60% (46–71) within >2–4 months, 57% (39–70) within >4–6 months, and 45% (22–61) within >6–9 months. In separate analyses by type and subtype, we estimated that vaccine effectiveness declined by 2–5 percentage points per month. Influenza vaccine effectiveness decreased during the 9 months after vaccination in children in Hong Kong. Our findings confirm the importance of annual vaccination in children. Influenz