Unlike most influenza viruses, the 2009 H1N1 virus affects many individuals between 5 and 40 years old, but very few 60 years old or older. It is not completely understood why, but scientists believe it may be because older people may have been exposed to other H1N1 viruses in their lives. H1N1 viruses were common until 1957, when they were eclipsed by H2N2 viruses.~In order to test susceptibility to the 2009 H1N1 virus, this study will examine how often people older than 60 get infected by the 2009 H1N1 live attenuated influenza vaccine (LAIV). The LAIV is made from a live virus that has been weakened, so the ability of the immune system to combat this weakened form is likely to mimic its ability to combat the normal form. Additionally, this study will examine how LAIVs work in older people; inactivated virus vaccines are used more often than LAIVs in older people, so little is known of LAIV's effects on this population.~Participation in this study will last 6 months. Participants will be people older than 60, divided into equal groups of people between the ages of 60 and 70 and people older than 70. Both groups will receive one dose of LAIV for 2009 H1N1 vaccine.~There will be six study visits, occurring at baseline and 2, 5, 7, 28, and 180 days after vaccination. Nasal wicks and throat swabs will be taken on Days 2, 5, and 7; a nasal wick alone will be taken at baseline and on Day 28. Blood samples will be taken at baseline and on Days 7 and 28. Participants will undergo physical exams at each visit.
Unlike most influenza viruses, the 2009 H1N1 virus has affected people between 5 and 40 years old more often than people 60 years old or older. It may be that older people have had greater exposure to previous strains of H1N1 influenza, and this previous exposure protects them from infection. This study will examine how older people respond to a version of the H1N1 virus vaccine that includes a live, noninfectious version of the virus.