ANN ARBOR, Mich.--- During a year in which the circulating strains of influenza showed genetic differences from the strains in vaccines, the traditional killed-virus flu shot was found to be effective in preventing influenza in healthy adults. The live attenuated-virus nasal spray vaccine also prevented illnesses but was less effective.
Both outcomes were determined by laboratory confirmation of flu infection, a University of Michigan study found.
Earlier studies had suggested that the nasal spray, sold by MedImmune as FluMist, might offer better protection against drifted viruses that had genetically changed between vaccine formulation and annual influenza activity. The nasal spray, which is based on a live but weakened virus, was 86 percent protective in one study conducted in children during a major drift year. However, FluMist had not previously been studied head-to-head against the shot in adults with laboratory confirmation.
The killed-virus flu shot is usually billed as 70 to 90 percent effective against circulating strains that are well matched to vaccine strains. During the 2004-2005 flu season, a University of Michigan team found that the killed-virus flu shot was 75 percent effective against a moderately-drifted type A virus and two types of B virus. The standard formulation of both the flu shot and the nasal spray vaccine includes two types of A influenza and one B, but in the 2004-05 season, there were two B strains circulating and one type A.
"On the other hand, FluMist was 48 percent effective. These results may only apply to the 2004-05 flu season," said Dr. Arnold S. Monto, professor of epidemiology. "In other years the results may be different. We need a more specific understanding of which viral changes matter and which don't. There are many things about vaccine protectiveness that we still don't completely understand."