The immunizing effect of influenza A/New Jersey/76 (Hsw1N1) virus vaccine administered intradermally and intramuscularly to adults.

2The immunogenicity and reactogenicity of inactivated influenza A/New Jersey/76 (Hsw1N1) whole-virus vaccine administered intradermally (40 chick cell-agglutinating units/0.1-ml dose) and intramuscularly (im; 200 chick cell-agglutinating units/0.5-ml dose) to human adults were evaluated. Among 18-24-year-old persons initially free of detectable antibody, intradermal vaccination induced lower titers of hemagglutination-inhibiting antibodies than did im vaccination, and a sequence of intradermal and im vaccinations did not offer any serologic advantage over one im dose. In contrast, persons over the age of 24 who initially lacked detectable antibody had as good a serologic response to intradermal vaccination as to im vaccination. Among individuals who had antibody before vaccination, immunization by either route induced greater increases in titers of hemagglutination-inhibiting antibody than occurred in initially antibody-negative persons, even when the latter were given two doses of vaccine. Somewhat fewer systemic reactions occurred after intradermal than after im vaccination, but the intradermal route was associated with local reactions in almost all vaccinees and with some residual pigmentary changes. These results suggest that intradermal vaccination should be used only in very selected circumstances.