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Redefining Influenza “Pandemic”

By Amesh A. Adalja, MD, May 20, 2011

It is commonly held that 6 influenza A pandemics have occurred during the past 125 years—in 1889, 1900, 1918, 1957, 1968, and 2009. All share characteristics that gave rise to an early definition of an influenza pandemic, which stipulates that an influenza outbreak is a pandemic if it meets 2 criteria:

  1. The outbreak originates in a specific geographic locale, spreads worldwide, and infects a large proportion of the population.
  2. The outbreak is caused by an influenza virus not related to or derived by mutation from the strains of influenza circulating immediately prior.

However, in a paper recently published in the Journal of Infection, authors C. Potter and R. Jennings argue that the traditional definition is inadequate because it excludes the 1933 and 1947 influenza outbreaks, both of which merit the designation of pandemic.

The 1977 H1N1 Experience

The outbreak and subsequent worldwide spread of the H1N1 virus in 1977 has generated controversy over its place among influenza pandemics. Although several features of this outbreak
do meet the traditional definition of a pandemic, others have raised questions regarding whether it was a true pandemic. For instance, the 1977 outbreak was almost exclusively limited to those under the age of 23 because at that time population immunity remained high to a similar strain that had circulated in the past. Further, the 1977 H1N1 strain was virtually identical to the 1947/1948 H1N1 strain, and the 1977 H1N1 strain did not displace the prior circulating strain of influenza, as happened in all prior pandemics.

Were the 1933 and 1947 Outbreaks Pandemics?

In 1933, a new variant of H1N1 influenza was identified in a patient in Puerto Rico. Analysis of the H1N1 influenza A viral strain revealed it to be serologically distinct from the seasonal variant previously circulating. The virus then spread to several continents, displacing the prior seasonal influenza strain. However, the 1933 outbreak has been overlooked as a pandemic, even though it was serologically distinct from the prior seasonal H1N1—like the 2009 H1N1 virus—and it spread worldwide.

The outbreak in 1947, which was initially considered a mild pandemic, shared the same characteristics. A serologically distinct H1N1 strain appeared in Australia and China before spreading worldwide and displacing the prior seasonal strain. Both of these influenza A H1N1 strains were derived via genetic mutations from the prior circulating seasonal influenza A H1N1s; therefore, they do not meet the existing definition of an influenza pandemic, despite their sero-distinctiveness, worldwide spread, and displacement of prior influenza strains.

Refined Influenza Pandemic Criteria

Potter and Jennings refine the definition of an influenza pandemic and add a third criterion:

  1. The outbreak must originate in a specific geographic locale with subsequent widespread dissemination.
  2. The hemagglutinin (HA) antigen of the influenza virus causing the outbreak is serologically distinct and does not cross-react with the prior circulating seasonal strain of influenza.
  3. Protective antibodies (HA) to the new virus are absent in a significant proportion of the population.

Applying this new definition, the influenza outbreaks of 1933, 1947, and 1977 would be counted among the other the long acknowledged pandemics.

Pandemic Severity Cannot Be Ignored

Even a mild pandemic can have serious consequences, as can the terminology and definitions that are used to describe a disease outbreak. Applying the refined definition developed by Potter and Jennings should serve to capture the essential characteristics of pandemics and inform planning.

However, a more inclusive definition does not render all pandemics equal. A severe, highly lethal pandemic, such as that of 1918, certainly merits a different level of response than does a milder pandemic, such as the 2009 H1N1 pandemic. It would be counterproductive to declare a pandemic without linking that declaration to actions that are specific to level of severity; and a “one-size-fits-all” response could, in the long run, compromise confidence in pandemic planning. Now that the definition has been refined, the next logical step in the evolution of thinking about pandemics may be to develop a way to accurately gauge and grade severity in order to prompt the appropriate level of response.


Potter CW, Jennings R. A definition for influenza pandemics based on historical records a definition for influenza pandemics [sic]. Journal of Infection 2011. Accessed May 19, 2011.