MedImmune acquired Aviron in January 2002 in an all-stock transaction valued at approximately $1.5 billion, gaining FluMist, the first intranasal live-attenuated influenza vaccine. The acquisition diversified MedImmune's portfolio beyond its lead product Synagis, and the company brought FluMist to the U.S. market in 2003. Wall Street reacted negatively to the near-term dilution, sending MedImmune shares lower on the news.