Immunizations - Smallpox

Viral Influenza

Influenza, commonly called the flu, is always caused by a virus.

Influenza, also known as the flu, is caused by a virus.

There are different strains of influenza:

Influenza A is the most widespread and most severe strain. It can affect both animals and humans. Influenza A is the cause of all the worldwide epidemics (pandemics) of the flu that have occurred. Influenza is responsible for over 200,000 hospitalizations a year in the U.S.
Influenza B infects only humans. It is less common than type A, but is often associated with specific outbreaks, such as in nursing homes. Flu caused by this strain tends to be milder than that caused by Influenza A.

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The 2009 H1N1 "Swine flu" pandemic was a new strain of influenza A.

Complications of the Flu. Pneumonia is the major serious complication of the flu and can be very serious. It can develop about 5 days after viral influenza. It is an uncommon event, however. It nearly always occurs in high-risk individuals, such as the very young or very old, pregnant women, and hospitalized or immunocompromised patients.

The largest number of H1N1 flu cases occurred in people ages 5 - 24. Fewer cases and deaths have been reported in people older than age 64. This is a different pattern than that seen with the seasonal or regular flu outbreaks.

[For more information, see In-Depth Report #94: Colds and the flu.]

Flu Vaccines

Description of Vaccines. The influenza vaccine is designed to provoke the immune system to attack antigens on the surface of the virus. (Antigens are molecules that the immune system recognizes as alien and targets for attack.)

Click the icon to see an image of antigens.

Unfortunately, the antigens in influenza viruses undergo genetic alterations (called antigenic drift) over time, so they are likely to become resistant to a vaccine that worked in the previous year. Vaccines are redesigned annually to match the current strain.

A vaccine for the H1N1 (swine flu) strain was released in the fall of 2009. This vaccine did not take the place of the seasonal flu vaccine, but was an additional vaccine available to those in high risk groups. A similar strain is now included in the 2010-11 seasonal flu vaccine.