Anti-NTHi antibodies in human infants. Infant sera (same infants analyzed in Fig. 1; n = 15) were analyzed by whole-cell ELISA for IgM antibodies against NTHi strain 2019 grown in Sia-free media (filled circles). Adult sera (n = 9) obtained from the pregnant mothers of the infants in this study were analyzed in parallel with infant sera for IgM anti-NTHi antibodies (open squares). Each circle or square represents the mean of triplicate values from one individual, quantified according to an IgM standard curve. Horizontal lines represent mean values for each group. Statistical analysis was performed using an unpaired two-tailed Student’s t test.