Immunity and cross‐protection to nephritis produced by Australian infectious bronchitis viruses used as vaccines

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Abstract
Four series of viruses were generated by passage through eggs incubated at 31°C. Initial evaluation of these viruses in SPF chickens showed that they had been successfully attenuated with regard to nephritis following between five and 25 egg passages. In addition, within each series viruses could be selected that minimised deaths from nephritis whilst optimising protective immunity against virulent challenge. Eight viruses were selected and used to vaccinate birds with maternal antibodies to IB. The viruses produced minimal mortalities and protected birds from subsequent challenge. Three viruses were selected from the latter trial and subjected to a cross protection experiment with three serologically distinct virulent challenge viruses: Ql/73, Nl/62 and N9/74. All the vaccine viruses protected birds from challenge while 2032/10 protected birds better than Vac 3/10 or Vac G/15. Finally, 2032/10 and Vac A3 were used in a similar experiment. Both proved to be mild, in terms of pathogenicity to the kidneys, and produced high levels of protective immunity against the three serologically distinct challenge viruses.