Understanding the Swine Flu Pandemic Severity
How the World Health Organization Determines the Phases of Infectious Disease
The WHO pandemic phases consider the geographical spread of a pandemic virus and are intended as a global call to countries to increase their alertness and readiness. Within each WHO phase, countries can assess national or regional pandemic severity to best use limited resources and interventions aimed at lowering pandemic-associated health impacts.
Criteria for a Pandemic
A pandemic occurs when a new influenza virus emerges to which people have no prior exposure. They, therefore, do not have immunity to it. Instead of 5–10 percent of the population becoming ill as with seasonal influenza, approximately 35 percent of people will become ill during a pandemic.
Because people have no immunity, the virus may cause more severe illness than is normal from seasonal influenza. The severity of a pandemic depends mostly on the virus that causes it, and severity may change over time during the course of the pandemic. While seasonal influenza causes death in less than 1 of 1,000 people who become ill, a pandemic virus may cause moderately higher to much higher rates of death. For the worst influenza pandemic we witnessed last century, the Spanish Flu of 1918, in the United States approximately 2 of every 100 who fell ill died. There were from 40–100 million deaths worldwide from the 1918 pandemic.
Severity for Pandemics in the 20th Century – Example numbers from the United States
| Severe pandemic | Spanish Flu (1918) — approximately 2 people died for every 100 that became ill |
| Moderate pandemic | Asian Flu (1957) — approximately 1-5 people died for every 1000 that became ill |
| Moderate pandemic | Hong Kong Flu (1968) — approximately 1-5 people died for every 1000 that became ill |
| Annual | Seasonal Flu (annual) — approximately 1 person dies for every 1000 that become ill1 in 1000 |
Assessment of pandemic severity is complex. Severity can vary from one pandemic to the next, from country to country, and among different population groups or geographical areas. Therefore, a single assessment of severity at the global level may not be relevant or helpful to countries. Second, severity will likely change as an event unfolds over time. As a result, monitoring is essential to detect changes in how the disease is developing. Third, the accuracy of a severity assessment will reflect the quality and availability of information about the virus and the people who are susceptible to infection. Such information is most limited at the beginning of a pandemic and takes some time to develop.
At the country level, three things will determine the "impact" of a pandemic on a population:
- (i) the pandemic virus and its characteristics
- (ii) the vulnerability of the population
- (iii) the capacity of the population for response
NB: This section was edited slightly from: Considerations for Assessing the Severity of an Influenza Pandemic. Weekly Epidemiological Record. Geneva: World Health Organization. 29 May 2009, vol. 84, 22 (pp 197–212) www.who.int/wer/2009/wer8422/en/index.html
Last Updated June 2009
