Scientists have been refining such cold-adapted flu viruses for over 30 years, since they were first isolated by Hunein F. Maassab at the University of Michigan School of Public Health in Ann Arbor. Like the injectable version, the nasal vaccine stimulates antibody production in the blood. In addition, it signals cells in the mucous lining of the upper airway to manufacture a kind of antibody that they alone can make. Both antibody types may be important in preventing flu, says Belshe. This should confer a more complete immune response than the currently used vaccine.
Influenza virus disguises itself with a new coat every year.
Consequently, scientists need a way to make a vaccine that's effective against each emerging strain. For the nasal vaccine, they mix the emerging virus with a master strain that grows at temperatures so low the wild virus can't reproduce. Scientists then grow the old and new viruses at low temperatures, thus disabling the wild strain, and inactivate the master strain by using compounds that bind to its surface. This leaves a virus that reproduces only under cool conditions and has the emerging strain's coat. He also points out that the investigators did not compare the nasal and injected vaccines.