though the virus in the live attenuated influenza vaccine is attenuated (low in virulence), it is still a living virus, and may cause an infection with complications in people with weakened immune systems or other underlying medical conditions. The live attenuated influenza vaccine is recommended only for people 2–49 years of age, and people who have a weakened immune system, pregnant women, and people with certain chronic diseases may not be eligible to receive LAIV. In contrast, inactivated virus vaccines contain no living virus, and cannot cause a live infection. Persons receiving the live attenuated influenza vaccine may shed small amounts of the vaccine virus during the first week. People coming in contact with the vaccinated person are not considered to be at risk, unless their immune systems are severely weakened (for example, bone marrow transplant recipients) and possible recombination with other (wild or live vaccine) flu strains. History The live attenuated influenza vaccine was originally developed by the University of Michigan School of Public Health in Ann Arbor, Michigan and later by Aviron, in Mountain View, California, under the sponsorship of the National Institutes of Health (NIH) in the 1990s. MedImmune, Inc. purchased Aviron in 2002, and the US Food and Drug Administration (FDA) approved the live attenuated influenza vaccine in June 2003. The FDA initially approved the live attenuated influenza vaccine only for healthy people aged 5 to 49 because of concerns over possible side effects. The live attenuated influenza vaccine is approved and recommended for healthy children 24 months of age and older. The FDA approved the unfrozen refrigerated version for the same age group (ages 5–49) in August 2006, following completion of phase III clinical trials. The cold-adapted influenza vaccine version of the vaccine is called CAIV-T, and is stable for storage in a refrigerator, rather than requiring freezer storage as did the originally-approved formulation. Approved for the 2007-2008 flu season, the refrigerated formulation can be distributed more economically, so that the price differential with shots (which had hampered sales of the original frozen version of FluMist) is now largely eliminated. FluMist was initially priced higher than the injectable vaccines, but sold only 500,000 of the four million doses it produced its first year on the market, despite a comparative shortage of flu vaccine in fall 2004. The price was sharply lowered the next year, and the company reported distributing 1.6 million doses in 2005. Because