Influenza

流行性感冒

Evidence-backed information EN 5/7 applicable sections · ZH 5/7 applicable sections Updated Oct 23, 2026

Influenza is an infectious respiratory disease caused by influenza viruses that circulates worldwide as seasonal epidemics and is preventable through vaccination and non-pharmaceutical measures. Antiviral medications, particularly neuraminidase inhibitors and baloxavir, can reduce the incidence of symptomatic influenza when used for post-exposure prophylaxis and treatment ^[1]^[2]^[3]^[4]. Annual vaccination remains the cornerstone of prevention, with effectiveness varying by age group, vaccine type, and season ^[2]^[3].

Read the full clinical and epidemiological profile 6

Definition

Influenza, commonly known as the flu, is an infectious disease caused by influenza viruses. Four types exist: A, B, C, and D, with types A and B circulating in humans and causing seasonal epidemics. Influenza A virus is widely distributed across aquatic birds, humans, pigs, and other mammals, while influenza B and C viruses primarily infect humans ^[4].

Clinical features

Symptoms range from mild to severe and commonly include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. Symptoms typically begin one to four days after exposure and last about two to eight days, with diarrhea and vomiting occurring more frequently in children. Complications can include pneumonia, acute respiratory distress syndrome, meningitis, encephalitis, and worsening of pre-existing conditions such as asthma and cardiovascular disease ^[2]^[4].

Epidemiology

Influenza imposes a substantial burden on community respiratory health, with children facing heightened risks of hospitalization and mortality during circulation periods ^[2]. The pathogen drives recurring seasonal epidemics and acute outbreaks, prompting public health authorities to implement structured intervention phases to limit spread and protect vulnerable populations ^[4].

Transmission

Influenza viruses are primarily transmitted through respiratory droplets produced by coughing and sneezing, with additional transmission occurring via aerosols and contact with virus-contaminated surfaces. Frequent hand washing, respiratory hygiene, and mask use reduce transmission risk ^[1]^[4].

Prevention

Annual vaccination is the primary prevention strategy, recommended for all eligible individuals beginning at six months of age without medical contraindications. When vaccines and antivirals are limited, non-pharmaceutical interventions such as hand and respiratory hygiene, self-isolation of symptomatic individuals, face mask use by patients and caregivers, surface disinfection, rapid testing, contact tracing, and social distancing measures including school closures and travel restrictions are endorsed for outbreak control. Antiviral post-exposure prophylaxis with zanamivir, oseltamivir, laninamivir, and baloxavir probably achieves important reductions in symptomatic influenza among those at high risk ^[1]^[2]^[3]^[4].

References

1 Zhao Y et al. Antivirals for post-exposure prophylaxis of influenza: a systematic review and network meta-analysis. Lancet. 2024 Aug 24. PMID: 39181596. doi: 10.1016/S0140-6736(24)01357-6. PubMed: https://pubmed.ncbi.nlm.nih.gov/39181596/
2 Committee on Infectious Diseases et al. Recommendations for Prevention and Control of Influenza in Children, 2024-2025: Policy Statement. Pediatrics. 2024 Oct 1. PMID: 39183669. doi: 10.1542/peds.2024-068507. PubMed: https://pubmed.ncbi.nlm.nih.gov/39183669/
3 Osterholm MT et al. Efficacy and effectiveness of influenza vaccines: a systematic review and meta-analysis. Lancet Infect Dis. 2012 Jan. PMID: 22032844. doi: 10.1016/S1473-3099(11)70295-X. PubMed: https://pubmed.ncbi.nlm.nih.gov/22032844/
4 Wikipedia contributors. Influenza - Wikipedia [Internet]. Wikipedia. cited 3 Sept 2026. Available from: https://en.wikipedia.org/wiki/Influenza