Most infl uenza vaccines used in the United States and Europe are produced in embryonated hens' eggs and are formaldehyde-inactivated preparations (11) . Because highly pathogenic infl uenza (H5N1) subtypes may kill embryonated eggs, use of viruses that are no longer pathogenic, such as H5 (which lacks the polybasic cleavage site), to reduce the virulence of infl uenza (H5N1) vaccine strains so that these can be effi ciently propagated in eggs for vaccine production is feasible (10) . Virus particles that lack the gene for the nuclear export protein or are defective for the matrix (M2) gene were used as live vaccines in animal models (12, 13) ; however, whether these replication-defective vaccines will work in humans is not known. Live attenuated (cold-adapted) infl uenza vaccines have long been used in Russia, and a similar product has been approved for use in the United States (14) . These vaccines will replicate in the host, and thus lower doses may be effective; however, the preexisting antibody to the virus is more likely to diminish the value of a live vaccine. Moreover, such live vaccines are reported to cause asthma-like reactivity in children (15) . Monitoring live infl uenza vaccines is important because the risk for reversion to pathogenicity remains.