Figure 2.

NA activity of A/OK/323/03 was reduced during passage at moi 0.01 but not at limiting dilution. The virus yield at each passage (TCIU) is compared to NA activity expressed as NA/HA ratio during limiting dilution passages (left) or moi 0.01 passages (right). Measurements were made on the first well for 0.01 moi passages and on the last well that showed infection for the limiting dilution passages. For comparison at the same scale, we plotted the ratio NA/(log₂HA) and log(TCIU).