What is the duration of contagiousness for someone with influenza A?

Duration of Contagiousness for Influenza A

Adults with influenza A are contagious from 1 day before symptoms begin through approximately 5-6 days after symptom onset, while children remain infectious for up to 10 days, and immunocompromised individuals may shed virus for weeks to months. 1

Standard Infectious Period by Population

Immunocompetent Adults

• The infectious period begins 1 day before symptom onset and continues through day 5-6 after symptoms appear 1
• Viral shedding decreases rapidly by days 3-5 after illness onset in most adults 1
• Most adults complete viral shedding by 5-7 days after illness onset 1
• Only 5% of transmission events occur more than 3 days after symptom onset 2
• The amount of virus shed directly correlates with infectivity and transmission risk 1

Children

• Children can be infectious before symptoms begin and remain contagious for up to 10 days after symptom onset 1
• Young children may shed virus several days before illness onset 1
• The prolonged shedding period in children necessitates extended isolation precautions compared to adults 1

Immunocompromised Patients

• Severely immunocompromised persons may shed virus for weeks or months 1
• Solid organ transplant recipients and other immunosuppressed individuals may be contagious for 10 or more days 3
• These patients require extended isolation precautions and may need documentation of negative testing before discontinuing precautions 3

Clinical Implications for Isolation

General Population

• Isolation measures should continue for at least 5-6 days after symptom onset in adults 1
• For children, maintain isolation for up to 10 days after symptom onset 1
• The incubation period averages 2 days (range 1-4 days) 1

Hospitalized Patients

• Continue isolation precautions until hospital discharge OR until signs and symptoms resolve AND the patient has been afebrile for at least 24 hours 3
• For immunocompromised hospitalized patients, consider maintaining droplet precautions until hospital discharge or documentation of negative H1N1 testing 3

Important Caveats

Peak Infectivity Timing

• Influenza A is most contagious shortly after symptom onset, with infectivity decreasing steadily over the course of illness 1
• Fever and respiratory symptoms correlate with the highest infectivity 1
• The mean duration of viral shedding in immunocompetent adults is approximately 5 days but may extend to 10 days or more in vulnerable populations 4

Post-Infectious Symptoms

• Cough and malaise can persist for more than 2 weeks even after the infectious period ends 1
• Uncomplicated influenza typically resolves after 3-7 days for most people 1
• Prolonged symptoms do not necessarily indicate continued contagiousness in immunocompetent individuals 1

Transmission Dynamics

• The virus spreads primarily through large respiratory droplets from coughing and sneezing 1
• Direct and indirect contact with contaminated surfaces or hands also contributes to transmission 1
• Significant viral particles are present in respiratory secretions during the infectious period 4