Figure 3. Limited amplification of challenge virus in the lungs of pre-infected mice.

Naïve mice received pre-infection (intranasal inoculation) or intramuscular injection of stated doses of PR8 influenza virus. 7-days apart, mice were challenged with 1.25 × 10⁴ PFU PR8 virus. Naïve mice with challenge only, served as controls. Virus titers in the lungs were measured by plaque assay on stated time points after the challenge. Data are representative of at least three similar experiments and presented as mean ± s.d. (n = 6/group).