How many days is a person with influenza contagious?

Influenza Contagious Period

Adults with influenza are contagious from 1 day before symptoms begin through approximately 5-6 days after symptom onset, while children can remain infectious for up to 10 days after symptoms start. 1

Contagious Period by Population

Healthy Adults

• Infectious from 1 day before symptom onset through day 5-6 after symptoms begin 2, 1
• 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
• The peak infectivity occurs shortly after symptom onset, with infectivity decreasing steadily over the course of illness 1

Children

• Can be infectious before symptoms begin and remain contagious for up to 10 days after symptom onset 2, 1
• Young children can shed virus several days before illness onset 1
• Isolation measures for children should be maintained for the full 10 days after symptom onset 1

Immunocompromised Patients

• May shed virus for weeks to months, requiring extended isolation precautions 2, 1
• This includes solid organ transplant recipients, HIV-positive patients, those on chemotherapy, and patients on high-dose corticosteroids 2
• Prolonged viral shedding necessitates longer isolation periods, potentially until hospital discharge or documentation of negative testing 2, 1

Patients with Severe Disease

• May experience 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

Clinical Implications for Isolation

Standard Isolation Recommendations

• For hospitalized patients, maintain Standard and Droplet Precautions until symptoms resolve and patient has been afebrile for at least 24 hours 2
• Due to the possibility of prolonged viral shedding and atypical symptoms, it is reasonable to maintain Droplet Precautions until hospital discharge if possible, or documentation of negative testing 2

Practical Isolation Duration

• Adults should isolate for at least 5-6 days after symptom onset 1
• Children require isolation for up to 10 days after symptom onset 1
• Immunocompromised patients need individualized assessment with potentially much longer isolation periods 1

Important Caveats

The amount of virus shed correlates directly with infectivity and transmission risk 1, meaning patients are most contagious when symptoms are most severe, particularly when fever and respiratory symptoms are prominent 1.

The incubation period is 1-4 days (average 2 days) 1, which explains why transmission occurs before symptom onset—infected individuals are already shedding virus during this pre-symptomatic phase 1.

Even after the infectious period ends, cough and malaise can persist for more than 2 weeks, but this does not indicate ongoing contagiousness 1.