Immunizations - Viral Influenza

FluVaccines

Description of Vaccines. Vaccines against the flu use inactivated (not live) viruses.The influenza vaccine iscommonly called a "flu shot." It is designed to provoke the immune system to attack antigens contained on the surface of the virus. (Antigens are foreign molecules that the immune system specifically recognizes as alien and so targets for attack.)

Click the icon to see an image of antigens.

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Unfortunately, the antigens in these 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 then redesigned annually to match the current strain.

Influenza A. The influenza A virus is further categorized by primary molecular antigens (hemagglutinin and neuraminidase), which serve as the targets for the vaccines. Influenza A is a particular problem because it can infect other species, such as pigs or chickens, and undergo major genetic reassortments.
Influenza B viruses tend to be more stable than influenza A viruses, but they too vary. Although influenza B has been far less common than A, a vaccine for type B is important because experts are concerned that small children will not have developed any immunity to the virus and will experience severe flu if they are exposed to type B.

Until recently the vaccine has been administered only with injection. A vaccine (FluMist) that can be delivered in a nasal spray has now been approved for people between5 and 49 years of age. The vaccine contains live viruses that have been engineered to replicate in the cool temperatures of the nasal passages, but not in the warmer lungs and lower airways. Its presence in the nasal passages boosts the specific immune factors in the mucous membranes that fight off the epidemic viruses. Studies in 2003 reported protection against the flu that ranged between 66% and 92%, depending on whether the flu was A or B. (The lower rates were those observed for influenza B, particularly a new variant.) In a broad 2004 Canadian study, one to two doses of the intranasal spray offered significantly more effective protection to children (aged 15 - 71 months) than injected vaccines. However, most insurance plans do not cover the cost of the intranasal vaccine. A preservative-free intramuscular injectable vaccine (Fluzone) is also now available.

Timing and Effectiveness of the Vaccine

Ideally, appropriate candidates should be vaccinated every October or November. However, it may take longer for a full supply of the vaccine to reach certain locations. In such cases, the high-risk groups should be served first.