Relationship between Antibody Titers and Protective Rate of Infection after Inoculation with Newcastle Disease Vaccine

Abstract

Studies were made on the relationship between hemagglutination-inhibiting (HI) and virus-neutralizing (VN) antibody titers and ability to protect against infection in birds immunized by single or simultaneous inoculation with live-virus and inactivated Newcastle disease vaccine.
1. There was a positive correlation between HI and VN antibody titers. When the average VN antibody titer was calculated from the HI antibody tietr, it was indicated by the following equation expressed with a straight line at the critical rate of 1 per cent by regression analysis: y=0.17+0.35x The average HI titer as calculated from the VN titer was indicated by the following equation expressed with a straight line at the critical rate of 5 per cent when the VN titer ranged from 0 to 2.5. y=0.19+0.23x When the VN titer was extended to a range from 2.6 to 3.5, the equation was not expressed with a straight line.
2. The higher the HI titer, the higher the protective rate of infection. When the HI titer was equal, the older were birds, the higher became this rate in the birds. When the antibody titer was equal and low (2>to 16), this rate was the highest in the group inoculated with inactivated vaccine alone, the second highest in that inoculated simultaneously with both vaccines, and the lowest in that inoculated with live-virus vaccine alone.
3. The relationship established between the VN titer and the protective rate of infection was essentially the same as that between the HI titer and this rate, There was no difference in this rate due to age of the bird.
4. There was a remarkable difference in the protective rate of infection between the vaccinated and non-vaccinated groups, even in the case of equal antibody titer. The rate was higher in the vaccinated groups.