From the Guidelines
Influenza is typically contagious for approximately 5 days after illness onset, with adults being infectious from the day before symptoms begin until 5 days after, and children potentially being infectious for a longer period, up to 10 days or more. Specifically:
- Adults are usually contagious from 1 day before symptoms start until 5 days after
- Children may be contagious for longer, up to 10 days
- People with weakened immune systems could potentially spread the virus for weeks or months, as noted in the study by 1 To reduce transmission:
- Wash hands frequently
- Cover coughs and sneezes
- Disinfect commonly touched surfaces
- Wear a mask if you must be around others while ill The flu virus spreads through respiratory droplets when coughing, sneezing, or talking, as explained in the study by 1. You're most contagious in the first 3-4 days of illness when viral shedding is highest, and even after fever subsides, you may still be contagious, so complete the full isolation period recommended by your doctor, based on the findings of 1 and 1.
From the Research
Duration of Influenza Contagiousness
- 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.
- Influenza transmission mainly takes place during the initial four days of the illness, and convalescence from disease should be expected to comprise seven days from symptom onset 3.
- Hospitalized patients with influenza A virus infection can shed detectable virus beyond the 5- to 7-day period traditionally considered the duration of infectivity 4.
- Most individuals shed less than 10% of their total shed virus before symptom onset, and shedding after isolation varied substantially across individuals, isolation end points, and infectiousness assumptions 5.
Factors Affecting Contagiousness
- Age: Children under 5 years experienced greater peak shedding, longer durations of shedding, and elevated influenza-like illness (ILI) symptom scores compared with other age groups 5.
- Vaccination status: Vaccination can reduce the risk of influenza transmission, but the effectiveness of the vaccine can be affected by antigenic drift and shift 6.
- Health status: Patients with underlying chronic medical conditions, immunosuppression, and other health issues may shed the virus for a longer period 4.