Fig. 3

Seroconversion to prime or booster vaccination in SPF chickens with high, medium or low titers of PTAs. 1-day-old SPF chickens were passively transferred with 0.3 mL of high (HI = 12 log₂) (a), medium (HI = 9 log₂) (b), or low (HI = 6 log₂) (c) titers of antibodies or PBS (d) per chicken via intravenous injection. Half of the 1-day-old chickens in each group were vaccinated. All were boosted 14 d after vaccination. Serum H9N2–specific antibody responses were measured weekly. The symbol (*) denotes differences between two groups at the same time point (* P < 0.05, ** P < 0.01, and *** P < 0.001)