Glossary of Flu Terms

Helpful terms to use when talking about influenza

Adjuvant: A substance added to a flu vaccine to increase the immune response to the vaccine.

Antigen: A protein located on the surface of an influenza virus that can cause your body to create an immune response. All flu vaccines contain antigens, which trigger an immune response to influenza.

Antigenic drift: An ongoing process that involves small changes in influenza virus genes, which can cause changes to the two antigens on the surface of the virus. Antigenic drift can cause flu viruses’ antigens to change so much that a person’s immune system might not recognize them, making that person susceptible to the flu.

Attenuated virus: Another way to describe live flu viruses that are weakened. This term may be used to discuss the use of flu viruses in nasal spray vaccine development.

Epidemic influenza: Heightened flu activity in a large region, such as a city or country. The annual flu season in the United States is an example of a seasonal epidemic.

Hemagglutinin (HA): An antigen found on the surface of a flu virus. 

Inactivated virus: Killed flu viruses that are used in making injectable flu vaccines.

Pandemic influenza: An event in which a new influenza virus spreads worldwide from person to person, resulting in the infection of large populations worldwide.

Quadrivalent: A term used to describe vaccines that are designed to provide protection against four viruses.

Seasonal influenza: The spread of influenza viruses and related illness during a specific time of year, which varies based on location in the Northern or Southern Hemisphere. In the United States, flu season usually begins in the fall and continues through the spring.