Fig. (3) Virus isolation from mouse lungs after sequential challenge with homologous and heterologous influenza viruses followed by SGB serotype II streptococcus infection. Groups of mice were immunized twice using 10⁶ EID₅₀ LAIV, GBSV or combined LAIV+GBSV; 3 weeks post-boost the mice were challenged with 500 MID₅₀ of the A(H7N3)-wt influenza virus or with 100 MID₅₀ of A/PR8/34 influenza virus and infected with 10⁷ CFU of type II GBS 24 hours later. Infectious virus was isolated in CE from lung homogenates obtained on 48 and 96 hours post infection (h.p.i.) with challenge viruses, and expressed as a log₁₀EID₅₀/ml; (*) - p=0.029, (**) - p= 0.028.