Influenza A (H5N1): Will It Be the Next Pandemic Influenza? Are We Ready?

Influenza experts have predicted the next pandemic flu for many years. What they fear most is an event like the Spanish flu of 1918 to 1919—the largest outbreak of fatal infectious disease during the past century. Avian influenza (influenza A [H5N1]) appears to have that potential. Dramatic response strategies have been undertaken in some countries, but the response in others has been far more measured. For example, the United Kingdom has committed to stockpile enough oseltamavir to treat 25% of its population in an effort to be prepared; the United States has enough in its Strategic National Stockpile for less than 1% (1). So how real is this risk?

Most influenza viruses occur in birds, particularly the aquatic waterfowl that are their natural reservoir. Only a few types of influenza virus have circulated widely in humans. “Bird flu” refers colloquially to both influenza in birds and to instances when these avian viruses jump the species barrier to cause human disease. The influenza A genome encodes 2 major surface glycoproteins, the hemagglutinin (H) and neuraminidase (N) proteins. The subtypes of these proteins are antigenetically distinct, having 16 H subtypes and 9 N subtypes. All of these subtypes may be found in birds, but only H1, H2, and H3 have caused pandemics and epidemics in people. Influenza viruses are constantly evolving into new antigenic variants, which accounts for vulnerability and the need for annual vaccination. Minor changes, or “drifts,” are the rule; major changes, or “shifts,” represent new hemagglutinin with or without new neuraminidase proteins that result in a novel virus for which the population lacks specific immunity.

As noted, “Spanish flu,” involving influenza A (H1N1), was the largest rapidly fatal pandemic in human history. The virus traversed the world in less than 1 year, causing at least 40 million deaths. …