The influenza A and B viruses that routinely spread in people are responsible for seasonal flu epidemics each year. Over the course of a flu season (which typically occurs between October and May), different types of influenza are passed from person-to-person, causing illness. Usually, vaccination with the seasonal influenza vaccine provides some protection against the strains of flu that are circulating at the time. CDC recommends that everyone 6 months and older get a seasonal flu vaccine each year.
A novel influenza virus is an influenza A virus with a subtype that is different from the flu viruses that usually spread in people (H3N2 and H1N1). Some examples include H7N9, and H5N1. Occasionally, strains of influenza that normally affect birds, pigs, and other animals can infect humans. When flu viruses that normally affect pigs (swine flu viruses) cause infections in humans, these viruses are called variant influenza viruses. One example of a variant flu virus is H3N2v.
Sometimes, human infections with novel or variant flu viruses occur because of the close contact between humans and animals. Other times, the infections occur because of changes in the influenza virus.
Flu viruses constantly change, and the changes can happen slowly over time or suddenly. Sometimes, the changes result in viruses that spread more easily from animals to humans. Flu viruses can change through antigenic drift or antigenic shift:
An influenza pandemic is a worldwide outbreak of disease. This occurs when an influenza virus undergoes an antigenic shift and creates a completely new subtype of influenza A with the ability to cause illness in people and spread easily from person-to-person.
We are not currently experiencing an influenza pandemic. However, two identified avian (bird) influenza viruses have the potential to cause a pandemic—H5N1 and H7N9. Because H5N1 and H7N9 do not circulate in humans, people have little to no immunity against these viruses. Human infections with these viruses have happened rarely, but if either virus changes in such a way that it is able to infect humans easily and spread easily from person-to-person, an influenza pandemic could result.
During an influenza pandemic, the novel (new) virus can spread rapidly across the world, causing: increased sickness and death; shortages in medical equipment/supplies (such as ventilators and antiviral medications) and hospital beds; event cancellations; and school and business closures.
Four influenza pandemics have occurred over the last hundred years. The 1918-1919 pandemic was the most severe, causing an estimated 50 million deaths worldwide. The most recent pandemic occurred in 2009-2010, with the H1N1 (swine flu) virus. The first U.S. case was diagnosed on April 15, 2009, and on April 26th, the U.S. government declared H1N1 a public health emergency. The CDC estimates that 43 million to 89 million people were infected with the virus between April 2009 and April 2010, and between 8,870 and 18,300 related deaths occurred.