Fig. 1.

Protective efficacy of influenza H1N1-inactivated vaccine against lethal challenge with 1918 HA/NA:WSN recombinant influenza virus. Groups of BALB/c mice received a single i.m. inoculation of H1N1 or H3N2 (X-31) vaccine. Control mice received PBS in place of vaccine. Twenty-two days after vaccination, mice were challenged i.n. with 100 LD₅₀ of 1918 HA/NA:WSN recombinant virus. Mice were monitored for survival (A) or killed 5 days later, and virus titers in individual lung, nose, and brain tissue were determined (B). Virus endpoint titers are expressed as mean log₁₀ eID₅₀/ml.