Influenza immunization in children and young adults: clinical reactions and total and IgM antibody responses after immunization with whole-virus or split-product influenza vaccines.

Clinical reactions and hemagglutinating-inhibiting (HAI) antibody responses to recent whole-virus and split-product influenza vaccines were studied in 168 children and young adults. The subjects initially received a monovalent vaccine, followed one month later by a trivalent preparation. The reactogenicity of whole-virus and split-product vaccines with an equivalent hemagglutinin content was similar except in the youngest age (6 to 36 months) group in which the whole-virus preparation was more reactogenic. The whole-virus vaccines were more immunogenic, especially in subjects who were previously unprimed (preimmunization HAI antibody titer, less than 5). In these subjects, the geometric mean titers of HAI antibody wee significantly higher after vaccination with whole-virus vaccines than the split-product vaccines. Specific IgM antibody was found more frequently after vaccination with whole-virus vaccines (34%) than after split-product vaccines (11%).