When is Influenza Infectious?
Influenza is contagious from approximately 1 day before symptoms begin through 5-6 days after symptom onset in adults, and up to 10 days in children, with immunocompromised patients potentially shedding virus for weeks to months. 1, 2
Infectious Timeline
Pre-Symptomatic Period
- Transmission begins 1 day before symptom onset, making pre-symptomatic spread a critical epidemiologic feature that complicates containment efforts 3, 1
- The incubation period averages 2 days (range 1-4 days) from exposure to symptom development 1, 2, 4
- Some infected individuals remain asymptomatic yet contagious throughout their infection 3
Peak Infectious Period
- Infectivity is highest shortly after symptom onset when fever and respiratory symptoms are most prominent 2, 4
- Viral shedding decreases rapidly by 3-5 days after illness onset in most adults 2
- The amount of virus shed directly correlates with transmission risk 2
Duration by Population
Healthy Adults:
- Contagious from 1 day before symptoms through approximately 5-6 days after symptom onset 1, 2
- Most adults complete viral shedding by 5-7 days after illness onset 2
- Isolation measures should continue for at least 5-6 days after symptom onset 2
Children:
- Can be infectious before symptoms begin and for up to 10 days after symptom onset 3, 1, 2
- Young children may shed virus several days before illness onset 2
- Isolation should be maintained for up to 10 days after symptom onset 2
Immunocompromised Patients:
- May shed virus for weeks to months, requiring extended isolation precautions 3, 1, 2
- This includes solid organ transplant recipients, patients on long-term corticosteroid therapy, and those with severe immunodeficiency 3, 2
- Isolation precautions should continue until hospital discharge or documentation of negative testing 3, 2
Clinical Implications for Isolation
Hospital Settings
- Implement Standard and Droplet Precautions immediately upon suspicion of influenza 3
- Healthcare personnel should wear surgical masks, nonsterile gloves, and gowns when caring for infected patients 3
- Place patients in private rooms with doors closed; if unavailable, cohort confirmed cases together 3
- Continue isolation until hospital discharge OR until symptoms resolve and patient has been afebrile for at least 24 hours 3, 2
- For immunocompromised patients, consider maintaining precautions until hospital discharge or negative testing 3, 2
Outpatient Management
- Patients should avoid contact with others, especially high-risk individuals, during the entire infectious period 2
- Healthcare workers with influenza-like illness should be removed from direct patient contact, with more stringent criteria for those in ICUs, nurseries, and transplant units 2
- Household contacts who are unimmunized may be candidates for antiviral prophylaxis 3
Important Caveats
Prolonged Shedding Risk Factors
- Patients with severe disease, extensive pneumonia, or respiratory failure may experience prolonged viral replication 2
- Those with chronic cardiopulmonary or neuromuscular conditions may have extended infectious periods 2
- Infants and very young children typically shed virus longer than older children and adults 2
Antiviral Treatment Considerations
- Early treatment with neuraminidase inhibitors (within 24-48 hours of symptom onset) may reduce duration of viral shedding, though the primary benefit is reducing symptom duration by approximately 24 hours 2
- To reduce transmission of drug-resistant virus, avoid contact between high-risk individuals and patients taking antivirals during therapy and for 2 days after discontinuation 2