Influenza viruses are constantly mutating and evolving, and new strains keep emerging. Because few people have immunity to a new strain—and because influenza spreads easily from person to person—new strains can travel quickly around the world. If a strain is particularly virulent, it may cause a pandemic, like the 1918-19 “Spanish flu,” which killed 20 million people, including 500,000 Americans.

The WHO International Influenza Surveillance Network, which includes 110 laboratories throughout the world (including a CDC-based WHO Collaborating Centre), gathers influenza isolates on all continents and collects data on new strains that have the potential for pandemic spread. In 1997, the government of Hong Kong made use of this network to identify a dangerous strain of avian influenza transmitted from chickens to humans that infected 18 persons and killed 6. The authorities feared that the strain (H5N1) might recombine with a human strain and become capable of human-to-human transmission and invited a CDC team to assist with control of the outbreak. Transmission stopped after the government of Hong Kong ordered the destruction of all chickens in Hong Kong that might be carrying the virus (see also Box 15).

This episode suggests that it may be possible to prevent influenza pandemics before they begin, or to mitigate the global impact of an influenza pandemic through early identification of a virulent strain and formulation of a strain-specific vaccine. What is required is continued international vigilance and cooperation (i.e., a global network) and—at the national level—the political will and resources to act on epidemiological and diagnostic evidence. Had the WHO network not been in place, or had the Hong Kong government been unable or unwilling to act, a virulent hybrid chicken/human strain of influenza for which virtually all people lack immunity—and for which there is no vaccine and few drug treatments—might have caused a massive global pandemic.