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Antibody titers in sera from mice. The log10 dilution of sera required to give an absorbance value at a cutoff of 0.5 was calculated for each individual serum sample. Mean values with standard errors are shown. Samples taken before challenge, which at the lowest dilution (20 times) gave an absorbance value of <0.5, were assigned a value of 1.30. P values for comparing sera taken before (pre) challenge and during infection against all three antigens are <10−4.
