Recombinant secreted haemagglutinin protects mice against a lethal challenge of influenza virus

Balb/c mice were immunized with 2 x 2 micrograms of purified recombinant secreted haemagglutinin, derived from the A/Victoria/3/75 (H3N2) virus. In the first immunization, Ribi adjuvant was used, while for the booster injection a monophosphoryl lipid A/muramyl dipeptide combination was chosen. Mice immunized in this way were 90-100% protected against a challenge with 20 LD50 of mouse-adapted, homologous virus (strain X47). Bromelain-solubilized haemagglutinin gave only 70% protection under comparable conditions.