2.6 Acid stability
PBS with different pH (5.0, 5.2, 5.4, 5.6, 5.8, 6.0 and 7.4) was mixed with virus-containing allantoic fluids and incubated for 30 min at 37°C. Then, tenfold serially dilution was conducted with PBS and 100 μl of diluted virus was transferred to 96‐well plates in duplicate. The virus titers were determined by 50% tissue culture infective dose (TCID50) on MDCK cells as described above.