For how many days is influenza infectious to others after symptoms start?

Infectious Period of Influenza After Symptom Onset

Adults with influenza are typically contagious from the day before symptoms begin through the fifth to sixth day after symptom onset, while children can be infectious for up to 10 days after symptoms begin. 1

Infectious Period by Age Group

• Adults: Contagious from 1 day before symptoms through approximately 5-6 days after symptom onset 1
• Children: Can be infectious before symptoms begin and for as many as 10 days after symptom onset 1
• Severely immunocompromised persons: May shed virus for weeks or months 1

Viral Shedding Patterns

• Viral shedding decreases rapidly by 3-5 days after illness onset in most adults 1
• Most adults complete viral shedding by 5-7 days after illness onset 1
• Young children can shed virus several days before illness onset 1
• The amount of virus shed correlates with infectivity and transmission risk 1

Special Populations with Prolonged Infectious Periods

• Immunocompromised patients: Can shed virus for weeks to months 1
• Patients with severe disease: Prolonged viral replication, especially those with comorbidities or receiving corticosteroid therapy 1
• Patients with chronic cardiopulmonary conditions: May experience prolonged symptoms and viral shedding 1
• Patients with certain neuromuscular conditions: May have extended infectious periods 1

Clinical Course and Infectivity

• Influenza incubation period is 1-4 days, with an average of 2 days 1
• Uncomplicated influenza typically resolves after 3-7 days for most people 1
• Cough and malaise can persist for more than 2 weeks even after the infectious period ends 1
• Fever and respiratory symptoms correlate with highest infectivity 1

Implications for Isolation and Prevention

• Isolation measures should continue for at least 5-6 days after symptom onset in adults 1
• For children, isolation should be maintained for up to 10 days after symptom onset 1
• For immunocompromised patients, longer isolation periods may be necessary 1
• Early antiviral treatment (within 48 hours of symptom onset) may reduce the duration and severity of symptoms, potentially reducing the infectious period 2

Common Pitfalls in Managing Infectious Period

• Underestimating pre-symptomatic transmission: Individuals can be contagious 1 day before symptoms appear 1
• Premature return to work/school: Ending isolation too early (before 5-6 days for adults or 10 days for children) can lead to transmission 1
• Relying solely on symptom resolution: Some individuals may still shed virus after symptoms resolve 1
• Failing to recognize prolonged shedding in high-risk groups: Immunocompromised patients require longer isolation 1

Understanding the infectious period of influenza is crucial for implementing effective isolation measures and preventing transmission in households, healthcare settings, and communities.