Severe complicated influenza

Severe complicated influenza is a contagious respiratory illness caused by influenza viruses that infect tissues in the nose, throat, and sometimes the lungs, and which can cause mild to severe illness and at times lead to death ^[1]. In the context of surveillance, severe complicated influenza refers to cases of influenza that are reported as severe and complicated, such as the 14,844 domestic and 63 imported severe complicated influenza cases reported in Taiwan from 2010 to 2020 ^[3].

Influenza typically begins suddenly with symptoms such as fever (though not universal), and some infected persons remain entirely asymptomatic ^[1]. In severe pediatric cases admitted to intensive care, common complications without CNS involvement included acute respiratory distress syndrome and pneumonia, while CNS-involved cases faced acute necrotizing encephalopathy with higher mortality ^[2]. Overall, severe complicated influenza carried a 37.5% mortality in CNS-involved children versus 6.25% without such involvement ^[2].

From 2010 to 2020 in Taiwan, 14,844 domestic and 63 imported severe complicated influenza cases were reported, with incidence peaking in 2019 at 19 to 99 per 1,000,000 persons and lowest counts in 2020 ^[3]. Most cases were male (57.3%), aged over 60 years (55.9%), occurred in winter (46.4%), and resided in Northern Taiwan (46.9%) ^[3]. A total of 1,672 deaths occurred (11.2% case fatality), and H1N1 was the dominant strain with China the main origin of imported cases ^[3].

Influenza viruses mainly spread through respiratory droplets generated by coughing, sneezing, or talking, and less commonly via contaminated surfaces or objects touched then transferred to mouth, nose, or eyes ^[1]. Infected individuals can transmit the virus before symptom onset as well as during illness, with an incubation period of about two days (range one to four days) ^[1].

Annual influenza vaccination has been shown to prevent flu and reduce severe complications leading to hospitalization or death, with vaccines updated yearly ^[1]. CDC also recommends non-pharmaceutical measures such as distancing from sick persons, covering coughs and sneezes, frequent handwashing, and improving air cleanliness to slow respiratory germ spread ^[1].

1. 1 US Centers for Disease Control and Prevention. About Influenza | Influenza (Flu) | CDC [Internet]. cited 23 Oct 2026. Available from: https://www.cdc.gov/flu/about/index.html
2. 2 Lin 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. 3 Wei 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/