Vaccines aren't supposed to cause disease. But that appears to be what's happening on Australian farms.
Scientists have found that two virus strains used to vaccinate chickens there may have recombined to form a virus that is sickening and killing the animals.
But when the researchers sequenced the genomes of the two new strains and the three vaccine strains, they found that the new viruses were actually stitched together from the European and Australian vaccines. Although it was not clear what mutations kept the vaccine strains from causing disease in the first place, they were probably lost when the viruses recombined, said Browning, whose team reports its findings online in Science.