Attenuation of influenza A/New Jersey/76 virus strain, preparation of live vaccine and vaccine reactogenicity in volunteers.

The NIB-3 strain (a recombinant of influenza A/New Jersey/76 and X-31 strains) was further attenuated by nine egg passages at 28 degrees C for the purpose of obtaining a "cold-adapted variant" suitable for preparation of live freeze-dried vaccine with 5% peptone. The vaccine with two infectivity titres, of 7 and 6 log EID50/dose respectively, administered intranasally, was studied in a double-blind placebo-controlled field trial. The reactogenicity of the vaccine was evaluated on the basis of fever reactions during the first three days after vaccination and by the detection of local virus recovery from throat swabs taken on the first four days after vaccination. The majority of vaccines had no fever. On the first day following vaccination the proportions of persons with low-grade fever (up to 37.4 degrees C) were the same in the groups given placebo and vaccine with 6 log EID50/dose respectively. In the group vaccinated with 7 log EID50/dose there were 8% more persons with low-grade fever than in the group given placebo, but this difference was not significant (P greater than 0.05). On the second and the third day after vaccination there was no difference between the proportions of persons with low-grade fever in all the three groups. The highest number of virus isolations, i.e. 26%, was found in persons vaccinated with 7 log EID50/dose, 72 h after vaccination. In the placebo group the virus was not isolated from any person during the entire course of the study.