COVID-19 is More Contagious Than Influenza
COVID-19 (SARS-CoV-2) is definitively more contagious than seasonal influenza, as demonstrated by its continued community spread despite public health measures that virtually eliminated influenza transmission during the same period. 1
Direct Evidence of Greater Contagiousness
The most compelling real-world evidence comes from the 2019-2020 respiratory virus season, when COVID-19 mitigation measures (masking, social distancing, hand hygiene) dramatically reduced influenza cases to near-zero levels in both hemispheres, yet COVID-19 continued widespread community transmission despite these same interventions. 1 This natural experiment definitively demonstrates that SARS-CoV-2 transmits far more efficiently than influenza virus.
- In northern hemisphere countries, the 2019-2020 influenza season ended 4-7 weeks earlier than typical, and the fall 2020 influenza season was virtually nonexistent. 1
- Southern hemisphere countries experienced drastically low influenza levels approaching zero cases after COVID-19 measures were implemented. 1
- Despite influenza being suppressed to negligible levels, COVID-19 maintained robust community spread under identical mitigation conditions. 1
Key Transmission Differences
The critical factor making COVID-19 more contagious is asymptomatic transmission—SARS-CoV-2 spreads efficiently through individuals without symptoms, while influenza patients are symptomatic in 93.4% of cases. 2 This means:
- Symptom-based screening effectively identifies most influenza cases but misses substantial COVID-19 transmission. 2
- COVID-19's basic reproductive number before interventions was 2.12, and isolation/quarantine alone reduced transmissibility by only 48.1%—insufficient to contain spread without additional measures. 3
- SARS-CoV-2 appears more contagious than the related SARS-CoV virus. 4
Clinical Impact Beyond Contagiousness
While contagiousness addresses transmission efficiency, the clinical consequences are equally important:
- In-hospital mortality for COVID-19 is 2.9 times higher than influenza (16.9% vs 5.8%), with an age-standardized mortality ratio of 2.82. 5
- COVID-19 patients more frequently develop acute respiratory failure, pulmonary embolism, septic shock, and hemorrhagic stroke compared to influenza patients. 5
- Even in children, where COVID-19 hospitalization rates are lower, in-hospital mortality is 10 times higher than influenza (1.1% vs 0.1% in adolescents). 5
Common Pitfalls
Do not assume that similar respiratory symptoms mean similar contagiousness—the transmission dynamics are fundamentally different. 2 The ability of COVID-19 to spread through asymptomatic carriers makes it far more difficult to control through traditional public health measures that work well for influenza. 2, 1