Immunity studies in calves vaccinated with a multivalent live respiratory vaccine composed of I.B.R., parainfluenza 3 and bovine adenovirus type 3.

The persistence of systemic and local antibodies was studied after two intranasal administrations of the vaccine, six weeks apart. Systemic antibodies to I.B.R. and adenovirus 3 evoked by the vaccine were still present 21 weeks following the second dose of the vaccine. Inconslusive results were obtained regarding the persistence of systemic and local PI-3 antibodies because of an intercurrent natureal PI-3 infection occurring during the observation period. Local antibodies to adenovirus type 3 were found in a high percentage of vaccinated animals 21 weeks after the second dose of the vaccine, whereas local antibodies to I.B.R. remained detectable in 50% of the animals eight weeks after thesecond dose. The results of a challenge study 21 weeks after revaccination show that the presence of local and systemic antibodies prevent the multiplication of PI-3 and BAV-3 in the upper respiratory tract. Protection against I.B.R. was achieved in the absence of detectable local antibodies.