Aviron filed for Food and Drug Administration approval of FluMist, the first nasal-spray vaccine shown to prevent the flu. If approved, FluMist would be the Mountain View-based biotechnology company's first product. Aviron requested approval for FluMist alone for use by children and adults and in combination with a flu shot for elderly patients. The product could generate as much as $400 million a year in U.S. sales. Aviron shares have surged in the last year. They fell 44 cents to close at $30.75 on Nasdaq.