Live Attenuated Influenza A Virus Vaccines in Children: Results of a Field Trial

One hundred three young children were inoculated intranasally with either influenza A/California/10/78 cold-recombinant vaccine (10(6.7) 50% tissue culture infective doses [TCID50] per child), CR-37 (H1N1), or influenza A/Washington/897/80 cold-recombinant vaccine (10(6.5) TCID50 per child), CR-48 (H3N2). The vaccine was well tolerated. Of the 51 children vaccinated with CR-37 (H1N1), 45 were initially seronegative for this virus; 33 of the 45 became infected with the vaccine virus, as indicated by a fourfold rise in antibody titer or by shedding of vaccine virus. Of the 52 children vaccinated with CR-48 (H3N2), six were initially seronegative and all were infected; 46 were initially seropositive and 25 of these children developed fourfold rises in antibody titer. An outbreak of influenza A (H3N2 type predominated) occurred in Huntington one to three months after the children were vaccinated. Significantly fewer febrile illnesses occurred in the CR-48 (H3N2) vaccine group than among the CR-37 (H1N1) vaccine group. Hemagglutination inhibition antibody (titer, greater than or equal to 1:32) to H3N2 was significantly associated with protection from illness and infection with influenza A/H3N2.