The influenza virus is not stable. In fact, it is in perpetual evolution, changing its genetic structure year by year. As a result, influenza demands an annual vaccine based upon expert guesswork. So every year, both the World Health Organization (WHO) and the U.S. Vaccine and Related Biological Products Advisory Committee meet to recommend which inactivated A and B viruses should make up the next season's vaccine. (WHO meets twice a year to cover both hemispheres.) The Virologists make their predictions based on the viruses currently in circulation.¹¹⁴

New influenza strains annually infect large numbers of people around the country, causing seasonal epidemics. Occasionally, a virus makes a significant change, by reassortment of novel genetic information, making the current vaccine powerless against it. Unimpeded, the novel strain travels around the world and infects millions of people causing a pandemic. Such a sudden genetic "shift" has caused three pandemics in the twentieth century (in 1918, 1957 and 1968). In 1997, virologists feared a possible fourth when a deadly avian flu killed six people in Hong Kong. They knew that rapid development of an effective vaccine looked difficult if not impossible, because fertilized eggs are used to manufacture vaccines. However, this virulent avian flu killed fertilized eggs. Fortunately, the avian strain seemed unable to spread by human-to-human contact.¹¹⁵