Preparing for pandemic flu

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The 2004-2005 influenza season thus far has been relatively mild in the United States, and public health officials hope current trends will persist through the end of the season.

However, a much greater flu threat worries world scientists and health officials -- a possible influenza pandemic caused by an emerging, highly virulent avian influenza virus.

What is pandemic influenza and how does it differ from the more familiar "interpandemic" or seasonal influenza we confront each year? Pandemic influenza is caused by an influenza virus so dramatically changed it is completely unfamiliar to our immune systems and can massively devastate immunologically "naive" populations. In contrast, seasonal influenza viruses change only slightly from year to year and most people harbor some residual immunity from prior exposure to identical or similar influenza viruses, which may be boosted by vaccination.

In the 20th century, mankind faced three influenza pandemics: the devastating 1918 "Spanish Flu" pandemic, as well as two less severe influenza pandemics in 1957 and 1968.

Today, health officials throughout the world fear the first influenza pandemic of this century is possible, because deadly new strains of avian influenza have emerged in Southeast Asia.

Most worrisome is the H5N1 influenza virus that first emerged in Hong Kong in 1997, infecting 18 people -- virtually all of whom had direct contact with domestic fowl. A massive culling of live birds prevented its further spread.

In the last two years, H5N1 has re-emerged with a vengeance. More than 50 confirmed human cases of H5N1 avian influenza have been reported since January 2004 in Vietnam, Thailand and Cambodia, and have resulted in the deaths of more than 40 people. In addition, the virus has killed millions of chickens and has expanded its host range by infecting other birds, pigs, tigers, leopards and domestic cats.

Unfortunately, we can only estimate the risk and timing of an influenza pandemic caused by the H5N1 virus by considering an escalating scale of what can best be described as "compounding probabilities."

To start with, there is no probability of a pandemic caused by H5N1 influenza if the virus does not exist in birds or other animals. It becomes slightly more probable if the virus appears in birds in a single country. It becomes slightly more probable again if it spreads to birds in many countries. Probability increases again if the virus begins to jump from birds to humans. A major increment in threat emerges in any human-to-human transmission. This is greatly compounded if the virus can efficiently spread from human to human.

In 2004, the H5N1 influenza virus spread to bird populations in 10 countries in Asia. Though the virus has jumped from birds to humans, it thus far has not acquired the ability to spread efficiently from human to human, though isolated cases of person-to-person transmission have been reported. If this form of transmission becomes widespread, a worldwide influenza pandemic is likely. Prevention will require a coordinated, multifaceted effort by agriculture and public health agencies, industry and biomedical researchers throughout the world.