Severe complicated influenza

Severe complicated influenza is a notifiable respiratory illness caused by influenza viruses that results in serious morbidity and mortality requiring hospitalization ^[1]. Influenza viruses infect tissues in the nose, throat, and sometimes the lungs, and can cause mild to severe illness that at times leads to death ^[3].

Influenza typically presents with sudden onset of symptoms, though not all patients develop fever and some infected individuals remain entirely asymptomatic, with one household study finding 8 percent of those testing positive had no symptoms ^[3]. Severe complicated cases may develop acute respiratory distress syndrome, pneumonia, or neurological complications such as acute necrotizing encephalopathy ^[2]. In pediatric patients, central nervous system involvement represents a particularly lethal manifestation, with encephalitis and encephalopathy associated with higher mortality rates compared to non-CNS cases ^[2]. The incubation period averages approximately two days with a range of one to four days ^[3].

Taiwanese surveillance data from 2010 to 2020 documented 14,844 domestic and 63 imported severe complicated influenza cases ^[1]. The incidence ranged from 19 to 99 per 1,000,000 individuals annually, peaking in 2019, with 1,672 deaths reported for an overall case fatality rate of 11.2% ^[1]. The majority of cases occurred in males (57.3%), in individuals aged over 60 years (55.9%), during winter (46.4%), and among residents of Northern Taiwan (46.9%) ^[1]. Symptomatic attack rates from community studies indicate children aged 0-17 years have the highest likelihood of infection at 9.3%, compared to 8.8% in adults 18-64 years and 3.5 times that of adults 65 years and older ^[3]. Influenza A virus subtype H1N1 was the dominant circulating strain during the study period ^[1].

Influenza viruses spread primarily through respiratory droplets produced when an infected person coughs, sneezes, or talks, with inhalation by nearby individuals representing the main transmission route ^[3]. Transmission can also occur indirectly when a person touches contaminated surfaces or objects and then touches their mouth, nose, or eyes ^[3]. Viral shedding and transmission potential exist both before symptom onset and during symptomatic illness ^[3].

Annual influenza vaccination is the primary prevention strategy and has been shown to reduce the risk of flu-related illnesses, severe complications, hospitalization, and death ^[3]. Flu vaccines are updated annually to match circulating viral strains ^[3]. Additional preventive measures include avoiding close contact with sick individuals, covering coughs and sneezes, frequent handwashing, and taking steps for cleaner air through ventilation measures ^[3].

1. 1Wei YH et al. Epidemiological characteristics of patients hospitalized with severe complicated influenza in Taiwan from 2010 to 2020: a retrospective study. BMC Infect Dis. 2026 Mar 10. PMID: 41808047. doi: 10.1186/s12879-026-13011-4.PubMed: https://pubmed.ncbi.nlm.nih.gov/41808047/
2. 2Lin CH et al. Comparison of severe pediatric complicated influenza patients with and without neurological involvement. Medicine (Baltimore). 2021 Apr 30. PMID: 33907160. doi: 10.1097/MD.0000000000025716.PubMed: https://pubmed.ncbi.nlm.nih.gov/33907160/
3. 3US Centers for Disease Control and Prevention. About Influenza | Influenza (Flu) | CDC [Internet]. cited 4 Sept 2026.Available from: https://www.cdc.gov/flu/about/index.html