Types of influenza viruses
The influenza A and B viruses that routinely spread in people cause seasonal flu epidemics each year. Over the course of a flu season (which typically occurs between October and May), different types of iflu are passed from person-to-person, causing illness. The seasonal influenza vaccine provides protection against flu viruses that are predicted to be circulating at the same time. CDC recommends that everyone 6 months and older get a seasonal flu vaccine each year.
A novel flu 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 flu 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.
How do influenza viruses change?
Flu viruses constantly change. 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:
- Antigenic drift is when changes to the flu virus happen slowly over time. It happens with both influenza A and influenza B viruses. The changes happen often enough that your immune system can’t recognize the flu virus from year to year. That is why you need to get a new flu vaccine each year.
- Antigenic shift only affects influenza A viruses and is a sudden, major change in the flu virus. This occurs when two flu viruses combine to form a virus with a new subtype or a mix of genes (including some from animals) that is very different. Because people have little or no immunity to the new virus, it can lead to a pandemic. An example of a “shift” occurred in the spring of 2009, when a new H1N1 virus with a new combination of genes (from pigs birds, and humans) emerged in people and quickly spread, causing a pandemic.
What is a flu pandemic?
A flu pandemic is a worldwide outbreak of disease. This occurs when a new flu A virus emerges that can cause illness in people and spread easily from person-to-person.
We are not currently experiencing an influenza pandemic. However, two identified avian (bird) flu 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 can infect humans easily and spread easily from person-to-person, an influenza pandemic could result. This is why public health conducts disease surveillance and preparedness planning.
What challenges could occur during a flu pandemic?
During a flu pandemic, the new virus can spread rapidly across the world, causing increased sickness and death; shortages in medical equipment, supplies (such as ventilators/breathing machines and antiviral medications) and hospital beds; even cancellations; and school and business closures.
When have flu pandemics occurred?
Four flu 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. For more information, visit CDC’s Pandemics webpage.
For more information on novel, variant, or pandemic influenza:
- Understanding Flu Viruses (CDC)
- H5N1 Bird Flu: Current Situation Summary (CDC)
- Avian Flu: What CDC Does
- H3N2v: Information for the General Public (VDH)
- H7N9: Information for the General Public (VDH)
Page last updated August 28, 2023