The efficacy of a recombinant rinderpest vaccine, constructed by inserting the rinderpest virus haemagglutinin gene into attenuated vaccinia virus, LC16mO strain, was tested in cattle. After subcutaneous inoculation of 10(8) plaque-forming units (pfu) of the recombinant vaccine, neither palpable skin lesions nor increases in body temperature were observed, indicating the absence of detectable clinical reactions. All the vaccinated cattle were completely protected from challenge with the Saudi 1/81 strain of virulent rinderpest virus. Contact control cattle housed in the same pen with the vaccinated animals did not develop antibodies to rinderpest or vaccinia viruses, and developed typical clinical signs of rinderpest after challenge with virulent rinderpest virus, indicating that there was no contact transmission of the recombinant virus. The 50 per cent protective doses of the vaccine, estimated by the mortality and morbidity rates respectively. were 10(4) and 10(5) pfu. To observe the effect of pre-existing immunity to vaccinia virus on the efficacy of the vaccine, cattle inoculated with the Lister strain of vaccinia virus three weeks earlier, were vaccinated with the recombinant virus. These animals developed antibodies to rinderpest virus and were protected from challenge with virulent rinderpest virus, showing that the vaccine was effective in animals already immune to vaccinia virus. The effectiveness and safety of the vaccine demonstrated in this study suggest that it has potential as a new vaccine against rinderpest.
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