Serological response of chickens vaccinated and challenged. Hemagglutination inhibition (HI) assay titers in vaccinated chickens at the indicated times following vaccination and challenge with homologous virus. HI titers were assessed at 14 days post-vaccination (dpv), at 21 dpv, and at 14 days post-infection (dpi). Vaccines were administered at 1, 0.1, or 0.01 dose. Vaccines were (A) rgKA435/22.214.171.124c in layer chickens, (B) rgES2/126.96.36.199c in layer chickens, (C) rgKA435/188.8.131.52c in breeder chickens, and (D) rgES2/184.108.40.206c in breeder chickens. Individual data points are shown along with means and standard errors.