HA acid stability of viruses after inoculation and transmission.

Transmission experiments were performed as in Table 2. Viral isolates from nasal swabs and washes of donor pigs, contact pigs, and ferrets were amplified one round in MDCK cells. The HA activation pH of the samples were measured using an acid inactivation assay. The ΔpH values corresponding to the ratio of the pH of the sample to that of the inoculum were used to estimate the activation pH. The means (± SD) of absolute pH values are displayed over time (A, B, and C) and by virus group (D, E, and F). One-way ANOVA analysis followed by a Tukey post-hoc test was performed to compare groups, and significance is indicated as follows: *P < 0.05, **P < 0.01.