If a new flu virus emerges as a result of antigenic shift, and is spread easily from person to person, it could quickly travel round the world causing serious illness, and death for millions of people. This is known as a flu pandemic.
Before World War I influenza was not considered a major public health threat, although about 31 pandemics of influenza-like disease have been documented since the year 1580.
Three pandemics have occurred in the last century:
The "Spanish flu" (A/H1N1) of 1918 was the catastrophe against which all other pandemics are measured. More than 40 million people died worldwide, including more than 500,000 in the US. The world has not seen a virus this virulent since then.
Subsequent pandemics were the "Asian flu" (A/H2N2) of 1957 and the "Hong Kong flu" (A/H2N2) of 1968, which claimed the lives of more than 1.5 million people. The resulting productivity losses and medical expenses were estimated at more than 32 billion US dollars. It is not predictable when the next pandemic will occur.
There is some evidence that the viruses that caused the above-mentioned pandemics originated from animals (1918 from swine, 1957 and 1968 from avian strains). In 1997 an avian strain of influenza A (H5N1) emerged as a cause of human influenza in Hong Kong.
Usually flu viruses move from chickens to pigs, and then to humans. The 1997 virus was the first to move directly from chickens to humans causing life-threatening conditions. Public health authorities in Hong Kong ordered the slaughter of all live chickens. Luckily, the virus has not developed the potential to cause a pandemic, as there has been no successful transmission from human to human.
The WHO has developed a plan for the global management and control of an influenza pandemic. Key elements of this plan are surveillance and identification of potential pandemic viruses, dissemination of relevant information, logistic and other support to health authorities, and facilitation of vaccine production and international distribution. Many national health authorities have already developed or are currently developing their own pandemic emergency response plans.