How long are individuals contagious with influenza A?

From the Guidelines

Contagious Period for Influenza A

Individuals with influenza A are typically contagious for 5 to 7 days after the onset of symptoms, but this can vary depending on the individual's health and the severity of the illness 1.

• The contagious period can start from the day before symptoms begin, with adults usually shedding the virus for approximately 5 days after illness onset 1.
• However, young children can be infectious for ≥10 days after the onset of symptoms and may also shed the virus before their illness onset 1.
• Severely immunocompromised persons can shed the virus for weeks or months 1.
• To be cautious, it's recommended to assume contagiousness for at least 7 days after symptoms start, and potentially up to 10 days in immunocompromised individuals or those with severe illness.
• Antiviral medications like oseltamivir (Tamiflu) or zanamivir (Relenza) may help reduce the duration of contagiousness if started within 48 hours of symptom onset, typically taken for 5 days.
• The amount of virus shed, and presumably infectivity, decreases rapidly by 3–5 days after illness onset in an experimental adult human infection model, with shedding completed in most persons by 5–7 days after illness onset 1.

From the Research

Duration of Contagiousness

The duration of contagiousness for individuals with influenza A can vary.

• The mean duration of influenza virus shedding in immunocompetent adult patients is around 5 days, but may continue for up to 10 days or more, particularly in children, elderly adults, patients with chronic illnesses, and immunocompromised hosts 2.
• Viral RNA concentration demonstrates a nonlinear decrease with time, and 26% of oseltamivir-treated and 57% of untreated patients had RNA detected at 1 week after symptom onset 3.
• The duration of contagiousness can be influenced by factors such as antiviral treatment, with oseltamivir started on or before symptom day 4 being independently associated with an accelerated decrease in viral RNA concentration and viral RNA clearance at 1 week 3.

Factors Affecting Contagiousness

Several factors can affect the duration of contagiousness, including:

• Age: Children, elderly adults, and immunocompromised hosts may shed the virus for a longer period 2.
• Comorbidities: Patients with major comorbidities may have higher viral RNA concentrations and slower viral clearance 3.
• Antiviral treatment: Oseltamivir started within the first 4 days of illness can enhance viral clearance 3.
• Immune status: Immunocompromised hosts may shed the virus for a longer period 2.

Subtype Variations

The characteristics of influenza, including transmissibility and severity, can vary depending on the disease subtype 4.

• The attack rate and incubation period of influenza A were 2.3-12.3% and 1.4 days, respectively 4.
• Different subtypes of influenza A virus, such as H1N1, H2N2, H3N2, H5N1, and H7N9, can have different indexes related to transmissibility and severity 4.