A single previous exposure to N9 can abolish the brief protective effect of N9. The low-dose inoculum (0.05 ID50) infected only a small fraction of the control group treated twice with PBS (first column), and an even smaller fraction was infected in the control group treated with 2% N9 just before delivering the inoculum (middle column). But a significantly larger fraction became infected in the test group (right-hand column), revealing that the N9 dose delivered 12-hour earlier abolished the protective effect of the N9 dose delivered just before the inoculum: P = 0.013 by Fisher's exact two-sided test.
Cone et al. BMC Infectious Diseases 2006 6:90 doi:10.1186/1471-2334-6-90