From the Guidelines
A person with Influenza A is contagious for approximately 5 days after symptom onset.
Contagious Period Details
- The contagious period can start 1 day before symptoms develop and last for around 5 days in adults 1.
- However, children can be contagious for more than 10 days, and young children can also shed the virus before their illness onset 1.
- Severely immunocompromised individuals can shed the virus for weeks or even months 1.
Transmission Modes
- Influenza A is primarily spread through respiratory droplet transmission, such as when an infected person coughs or sneezes in close proximity to an uninfected person 1.
- The virus can also be spread through direct or indirect contact, such as touching contaminated surfaces or shaking hands with an infected person 1.
From the Research
Contagious Period of Influenza A
- The contagious period of Influenza A is not explicitly stated in most studies, but it can be inferred from the duration of viral shedding and the infectious period.
- A study published in 2012 2 estimated that the infectious period of Influenza A lasts approximately 1.0 day on average, with only 5% of cases being infectious for more than 2.9 days.
- Another study from 2007 3 found that hospitalized patients with Influenza A virus infection can shed detectable virus beyond the 5- to 7-day period traditionally considered the duration of infectivity, with 54% of patients still positive by PCR and 29% by culture at or beyond 7 days after symptom onset.
- However, it is essential to note that the duration of contagiousness may vary depending on factors such as the patient's age, health status, and the presence of underlying chronic medical conditions.
Factors Affecting Contagiousness
- The study from 2018 4 reviewed the transmissibility and severity of Influenza by subtype and found that the attack rate and incubation period of Influenza A varied depending on the subtype.
- The 2019 study 5 emphasized the importance of early treatment with anti-influenza drugs, which can decrease the duration of illness and reduce the risk of serious complications.
- The 2024 review 6 highlighted the need for antivirals targeting other areas of Influenza A virus replication to overcome antiviral resistance and combat yearly epidemics.
Duration of Contagiousness
- Based on the available evidence, it appears that the contagious period of Influenza A can last from a few days to up to 7 days or more after symptom onset, depending on the individual and the subtype of the virus.
- The study from 2012 2 suggested that four days of isolation may be enough to avoid most transmissions, but this may not be sufficient for all cases, particularly those with underlying health conditions.