BACKGROUND: Influenza remains an important cause of pediatric morbidity and mortality. Immunisation is critical to prevent hospitalisation and severe disease. The COVID-19 pandemic had far-reaching effects on influenza epidemiology and vaccine use.
METHODS: Participants were recruited prospectively from sentinel hospital sites (2019-2024). Children hospitalised with an acute respiratory illness (ARI) and laboratory-confirmed influenza were considered cases whilst those testing negative were considered controls. Vaccine effectiveness was estimated from the adjusted odds ratio of vaccination in cases and controls.
RESULTS: There were 8762 hospitalised influenza cases and 10,955 influenza-test-negative controls in children <16 years. Of cases, 25.0% were < 2 years of age, 38.4% had underlying medical risk factors and 6.9% were First Nations children. 7.1% required intensive care unit admission and 0.32% died. COVID-19 restrictions dramatically impacted on influenza activity: 12 cases were identified in 2020-2021, compared to 1557-2818 cases per annum in 2019 and 2022-2024 respectively. Antivirals were used in only 26.5% of cases. Reduced vaccine coverage following the COVID-19 pandemic was observed with 19.6% of test-negative controls vaccinated in 2022 compared to 43.9% in 2019. Reduced coverage persisted across all states, age groups and in those with medical comorbidities. Estimated vaccine effectiveness ranged from 56.3% (95%CI:46.7,64.2) in 2019 to 82.5% (76.6,86.9) in 2023. Effectiveness was demonstrated in all age groups and in those with and without medical risk factors.
CONCLUSION: Reduced influenza vaccine coverage was observed Australia-wide in 2022-24 compared to 2019. Despite this, influenza vaccine remains an effective influenza-prevention tool. Strategies to improve vaccine coverage are urgently required.