Immunization of infants and young children with live attenuated trivalent cold-recombinant influenza A H1N1, H3N2, and B vaccine.

Seventeen triply seronegative infants and young children, and 15 infants and young children seropositive to all three influenza virus strains were vaccinated intranasally with 10(5) TCID50 of each of three (H1N1, H3N2, and B) live attenuated, cold-adapted influenza vaccine strains. Seventeen controls were given placebo. Vaccination with trivalent influenza vaccine was not associated with adverse reactions in triply seronegative or seropositive children. Overall, 12 (71%), 13 (76%), and 13 (76%) of seronegative children were infected by H1N1, H3N2, or B vaccine viruses, respectively, as indicated by viral shedding or by hemagglutination inhibition assay or ELISA antibody response. Of the triply seronegative children, 47% shed all three viruses; 7 children had an antibody rise to all three vaccine viruses and 4 shed all three viruses. A dose of 10(4.4) - 10(5.0) TCID50 of each of three intranasally administered vaccine viruses was safe, immunogenic and antigenic; however, strategies to increase the proportion of children infected by each of the vaccine viruses should be studied, including higher doses and multiple doses of live trivalent vaccine.