Genotypic stability of cold-adapted influenza virus vaccine in an efficacyclinical trial

An investigational live influenza virus vaccine, FluMist, contains three co ld-adapted H1N1, H3N2, and B influenza viruses. The vaccine viruses are 6/2 reassortants, in which the hemagglutinin (HA) and neuraminidase (NA) genes are derived from the circulating wild-type viruses and the remaining six g enes are derived from the cold-adapted master donor strains. The six genes from the cold-adapted master donor strains ensure the attenuation, and the HA and NA genes from the wild-type viruses confer the ability to induce pro tective immunity against contemporary influenza strains. The genotypic stab ility of this vaccine was studied by employing clinical samples collected d uring an efficacy trial. Viruses present in the nasal and throat swab speci mens and in supernatants after culturing the specimens were detected and su btyped by multiplex reverse transcriptase (RT)-PCR Complete genotypes of th ese detected viruses were determined by a combination of RT-PCR and restric tion fragment length polymorphism, multiplex RT-PCR and fluorescent single- strand conformation polymorphism, and nucleic acid sequencing analysis. The FluMist vaccine appeared to be genotypically stable after replication in t he human host, All viruses detected during the 2-week postvaccination perio d were shed vaccine viruses and had maintained the 6/2 genotype.