A closer look at the flu bug

Influenza is a single-stranded, helically-shaped RNA virus that is categorized as one of three basic antigenic types: A, B, and C. Influenza A infects humans and other animals, causing moderate to severe illness among all age groups. Influenza B generally causes milder symptoms than type A and mainly affects children. Influenza C is rarely reported as a cause of human illness and has not been associated with epidemic disease. [2] Seasonal flu vaccines are developed using strains of Influenza A(H1N1), A(H3N2), and Influenza B viruses. [8]

How does the flu virus change and mutate over time?

The surface of an influenza virus contains two proteins: hemagglutinin (H) and neuraminidase (N). Over time, these proteins can change and evolve, making the influenza virus tougher to prevent and control. These changes are called either antigenic drift, or antigenic shift. [8]

In antigenic drift, small changes in the virus happen continually over time to produce new virus strains that may not be recognized by the body’s immune system. In most years, the influenza vaccine is updated with up to three new virus strains to keep up with these changes. This is why people need to get a flu shot every year. [8]

With an antigenic shift, an abrupt, major change in the influenza A virus results in new hemagglutinin and/or neuraminidase proteins, producing a new influenza A subtype. Antigenic shift mutations leave most people with little or no protection against the new virus. While it’s important to note that antigenic shift happens only occasionally, when it does it can have dramatic consequences: H5N1 (bird flu) is a result of antigenic shift. [8]

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