Background: Severe complications of influenza in children are uncommon but may result in admission to hospital or an intensive care unit (ICU) and death.

Methods: Active prospective surveillance using the Australian Paediatric Surveillance Unit with monthly reporting by pediatricians of national demographic and clinical data on children with <15 years of age hospitalized with severe complications of laboratory-confirmed influenza during ten influenza seasons 2008-2017.

Results: Of 722 children notified, 613 had laboratory-confirmed influenza and at least one severe complication. Most (60%) were <5 years of age; 10% were <6 months, hence ineligible for vaccination. Almost half of all cases were admitted to ICU and 30 died. Most children were previously healthy: 40.3% had at least one underlying medical condition. Sixty-five different severe complications were reported; pneumonia was the most common, occurring in over half of all cases. Influenza A accounted for 68.6% hospitalizations; however, influenza B was more often associated with acute renal failure (P = 0.014), rhabdomyolysis (P = 0.019), myocarditis (P = 0.015), pericarditis (P = 0.013), and cardiomyopathy (P = 0.035). Children who died were more likely to be older (5-14 years), have underlying medical conditions, be admitted to ICU, and have encephalitis, acute renal failure, or myocarditis. Only 36.1% of all children reported received antiviral medications, and 8.5% were known to be vaccinated for seasonal influenza.

Conclusions: Severe influenza complications cause morbidity and mortality in children, which may increase if coinfection with COVID-19 occurs in the 2020 season and beyond. Increased vaccination rates, even in healthy children, early diagnosis and timely antiviral treatment are needed to reduce severe complications and death.