Intranasal Vaccination
The CDC classifies FluMist as a "live attenuated influenza vaccine," which means that it is a weakened form of the actual influenza virus. An aerosolized suspension of the weakened virus is inhaled, and infection results when the virus crosses the mucosal lining of the nose, just as it does with "real" influenza infection. People with runny nose or nasal congestion from other infections or allergies may "wash away" FluMist before it has a chance to cross the nasal membranes. In people with asthma or a history of wheezing episodes, FluMist may exacerbate these symptoms.
Antibody Response
Cells infected with viruses express markers on their surfaces called "antigens." The immune system recognizes these antigens as abnormal, and cells called B cells make antibodies to attack them. Unfortunately, the process takes a few weeks and usually, by the time the B cells have produced enough antibodies, the infection has already been cleared by other cells in the immune system. Pre-exposure to the weak vaccine virus usually results in antibody production without causing illness. According to experts at the National Institutes of Health, many of the same people who have trouble fighting influenza infection also have trouble making antibodies against it. Therefore, FluMist is only recommended for healthy people between two and 49 years of age.
Antigenic Shift and Antigenic Drift
According to 2008 article in the journal "Vaccine," viruses such as influenza are constantly mutating, a phenomenon scientists dub "antigenic drift" and "antigenic shift." This affects the antigens the virus produces and how it displays them. To put it in different terms, viral antigens are like flags on a ship. Sometimes the virus will change its flag, other times the virus will display the same flag, but it will be slightly rolled up or have new parts added. The effectiveness of vaccines like FluMist depend on the antibody-antigen match, and some years that match is better than others.
References
- Centers for Disease Control and Prevention: Live Attenuated Influenza Virus
- Vaccine; The Biology of Influenza Viruses; N.M. Bouvier, P. Palese; September 12 2008
- National Institutes of Health Influenza Vaccine Patient Information
