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Flu Vaccine (cont.)

Effectiveness of Seasonal and Pandemic Flu Vaccines

Flu vaccine effectiveness is judged by the ability of the vaccine to generate an immune response (measured by a blood test known as a hemagglutination-inhibition assay; protection is deemed effective if assay titer of antibodies reaches 1:40 or greater). This applies to both seasonal and pandemic flu vaccines. A titer of 1:40 is considered to be protective 21 days after vaccination. Vaccine efficacy also varies from one person to another; one person's titer could be different from another person's titer, even if both were given vaccine from the same batch.

Previous studies of healthy young adults have shown influenza seasonal vaccine to be 70%-90% effective in preventing illness. In the elderly and those with certain chronic medical conditions such as HIV, the vaccine is often less effective in preventing illness. However, studies show the vaccine reduces hospitalization by about 70% and death by about 85% among the elderly who are not in nursing homes. Among nursing-home residents, vaccine can reduce the risk of hospitalization by about 50%, the risk of pneumonia by about 60%, and the risk of death by 75%-80%.

However, experience with the vaccine, studied each year, can show a wide variation in effectiveness. The 2014-2015 vaccine demonstrated this point. The vaccine was designed to protect against those major flu viruses that emerged in 2014. Unfortunately, new strains arose in late 2014 so that the vaccine synthesized for the 2014-2015 season was only about 23% effective. However, the CDC still recommended utilization of the vaccine as it seemed to reduce the severity of the flu in people who were vaccinated but still became infected with the flu virus.

Nasal-spray vaccines can provide effective protection to individuals 2-49 years of age. The effectiveness is less in the older population. The adequate administration and the low (0.6%-2.4%) but possible potential for spread to another person makes this vaccine restricted to healthy individuals 2-49 years of age.

Effectiveness and safety of vaccines are continuously being monitored by the cooperative efforts of the FDA, CDC, and other institutions in the U.S.

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