How long are you contagious with influenza A (flu A)?

How Long Are You Contagious with 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

Contagious Period by Population

Healthy Adults

• You become infectious 1 day before any symptoms appear, making pre-symptomatic transmission a critical feature of influenza spread 2, 1
• Peak contagiousness occurs during the first 3-5 days after symptom onset, when viral shedding is highest 1
• Most adults complete viral shedding by 5-7 days after illness begins, with infectivity declining rapidly after day 3-5 1
• The standard isolation period should be at least 5-6 days from symptom onset 1

Children

• Children can be infectious before symptoms begin and remain contagious significantly longer than adults 1
• The infectious period extends up to 10 days after symptom onset in pediatric patients 2, 1
• Young children may shed virus several days before illness onset, making them particularly efficient transmitters 1
• Isolation for children should be maintained for the full 10-day period 1

Immunocompromised Patients

• Severely immunocompromised individuals may shed virus for weeks to months, requiring extended isolation precautions 2, 1
• This includes solid organ transplant recipients, patients on immunosuppressive therapy, and those with primary immunodeficiencies 2
• For hospitalized immunocompromised patients, maintain droplet precautions until hospital discharge or documentation of negative influenza testing 2, 1
• The relatively short duration of positive assays (often 5-7 days from symptom onset) in immunocompetent patients does not apply to this population 1

Clinical Timeline

Incubation Period

• The incubation period is 1-4 days, with an average of 2 days from exposure to symptom onset 1, 3
• You are already contagious during the final day of this incubation period, before you feel sick 1, 3

Symptom Duration vs. Infectious Period

• Uncomplicated influenza typically resolves after 3-7 days in most people 1, 4
• However, cough and malaise can persist for more than 2 weeks, well beyond the infectious period 1, 3
• The presence of ongoing symptoms does not necessarily mean you remain contagious after day 5-6 in healthy adults 1

Isolation Recommendations

Home Isolation

• Stay home and avoid contact with others for at least 5-6 days after symptom onset if you are a healthy adult 1
• Children should remain isolated for 10 days from symptom onset 1
• You should also be fever-free for at least 24 hours without fever-reducing medications before ending isolation 2

Hospital Settings

• Isolation using Standard and Droplet Precautions should continue until hospital discharge or until symptoms resolve and the patient has been afebrile for at least 24 hours 2, 1
• Healthcare personnel wear surgical masks, gloves, and gowns when caring for influenza patients 2
• Private rooms with closed doors are strongly recommended 2

Important Caveats

Correlation with Symptoms

• Fever and respiratory symptoms correlate with highest infectivity, so the period when you feel worst is when you are most contagious 1
• The amount of virus shed directly correlates with transmission risk 1

Effect of Antiviral Treatment

• Early treatment with neuraminidase inhibitors (within 24-48 hours of symptom onset) may reduce the duration of viral shedding, though the primary benefit is reducing symptom duration by approximately 24 hours 1
• 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 1

Special Circumstances

• Patients with severe disease, extensive pneumonia, or respiratory failure may experience prolonged viral replication 1
• Those receiving corticosteroid therapy may have extended infectious periods 1
• Patients with chronic cardiopulmonary or neuromuscular conditions may experience prolonged viral shedding 1