Newcastle Disease Vaccination of Broilers with High- and Low-Titered Commercial Vaccines

SUMMARY. The virus titers of seven commercial B1 strain Newcastle disease vaccines were evaluated. A 2 log difference in virus content was found between the vaccine with the highest titer (108-8 EID50/ml) and the one with the lowest titer (106-8 EIDso/ml). Broiler chickens were vaccinated with the high- and low-titered vaccines to compare hemagglutination-inhibition (HI) antibody and challenge responses. The effect of vaccination at different ages on the HI titers was also examined. There were no significant differences between vaccine groups in HI antibody response or resistance to challenge. However, the high-titered vaccine may provide a margin of safety with the currently used methods of mass vaccination.