An influenza pandemic is a worldwide outbreak of infection. This occurs when a new influenza A virus subtype emerges that has never circulated among people or has not circulated among people for a long time. No one will have immunity to this virus. To cause a pandemic the new subtype must be able to spread easily from person to person. This is different from seasonal outbreaks that are caused by influenza subtypes that are already in circulation.
There were three pandemics during the 20th century. The 1918-19 Spanish Flu, the 1957-58 Asian Flu, and the 1968-69 Hong Kong Flu . During the Spanish Flu pandemic more than 500,000 people died in the US and more than 50 million people died worldwide. Many people died within a few days after infection and nearly half of those were young healthy adults. The Asian and Hong Kong flu pandemics were caused by viruses containing a combination of genes from human and avian influenza viruses. The Spanish flu virus seemed to have an avian origin.
Scientists believe that there will be another influenza pandemic and that it is only a question of when this will occur. No one knows how severe it will be but even a moderate pandemic could cause 200,000 deaths in the US, and millions more to become sick. Such a pandemic could overwhelm our nation's health care system and cause a severe economic impact. A vaccine would probably not be available in the early stages of a pandemic due to production time. The four different antiviral medications currently available to treat or prevent influenza may not be effective because influenza virus strains can become resistant to them.
The Centers for Disease Control and Prevention and the World Health Organization are monitoring influenza around the world, searching for possible pandemic strains. There is currently a concern about the H5N1 avian influenza causing human infection in Asia. The current mortality rate for human H5N1 infection in Asia is about 50%. Federal, state and local public health organizations are working together to be able to respond effectively to an influenza pandemic.