Influenza World Health Care Professionals

Influenza is highly infectious and is spread from person-to-person, most commonly by aerosolised respiratory droplets, laden with virus particles, which are produced by coughing or sneezing. It is also possible to become infected with influenza by direct contact with the virus – from shaking hands, touching infected handkerchiefs, doorknobs, telephones etc. – and then touching the mucosal surfaces of the nose or mouth.

Infection is considered to start in the upper respiratory tract and then spreads. There is histological evidence to suggest that influenza virus replication may occur throughout the entire upper and lower respiratory tract [1]. As viral replication increases (1 or 2 days after infection) and cytokine elaboration occurs, the patient experiences a sudden onset of symptoms.

Why are influenza outbreaks so explosive?

• Relatively short incubation period – usually 2–3 days (range 1–7 days) [1]
• High levels of virus in secretions
• Relatively long infectious period

- Adults may shed virus, and therefore remain infectious, for up to 6 days. [1]

- Children may shed virus, and therefore remain infectious, for 7–10 days [2] 

• Only small amount of virus is needed to initiate infection

Children and closed communities – key vectors for rapid transmission

As children tend to be naïve to influenza viruses they exhibit a higher degree of viral shedding (both in terms of level and duration) making them highly infectious. Furthermore as children come into close contact with large numbers of other children – such as in schools or daycare centres – the virus can be spread easily. Together, these factors indicate children to be key vectors for transmission of influenza.

Statistics suggest, that when one family member develops influenza, others in the family have a 1 in 5 chance of becoming infected with the virus. [3] Moreover, families with school-aged children are found to be twice as likely to be exposed to influenza than those without children in this age range. [4]

Influenza is also rapidly transmitted within closed communities, with average attack rates in nursing homes reported to be 43% during epidemics and 16% at other times. [5]

To view a short film about the transmission of influenza, follow the link.
To view a short film about the how influenza viruses infect cells, follow the link.

1. Nicholson K. Human influenza. In: Nicholson K, Webster RG, Hay A, eds. Textbook of influenza. Oxford, UK: Blackwell Science, 1998: 219–64.
2. Sato M, Hosoya M, Kato K, Suzuki H. Viral shedding in children with influenza virus infections treated with neuraminidase inhibitors. Pediatr Infect Dis J 2005; 24: 931–2.
3. Welliver R, Monto AS, Carewicz O, et al. Effectiveness of oseltamivir in preventing influenza in household contacts: a randomized controlled trial. JAMA 2001; 285: 748–54.
4. Taber LH, Paredes A, Glezen WP, Couch RB. Infection with influenza A/Victoria virus in Houston families, 1976. J Hyg (Lond) 1981; 86: 303–13.
5. Patriarca PA, Arden NH, Koplan JP, Goodman RA. Prevention and control of type A influenza infections in nursing homes. Benefits and costs of four approaches using vaccination and amantadine. Ann Intern Med 1987; 107: 732–40.