Leaky Vaccines Enhance Spread of Deadlier Chicken Viruses

Over the past fifty years, Marek’s disease—an illness of fowl—has become fouler. Marek’s is caused by a highly contagious virus, related to those that cause herpes in humans. It spreads through the dust of contaminated chicken coops, and caused both paralysis and cancer. In the 1970s, new vaccines brought the disease the under control. But Marek’s didn’t go gently into that good night. Within ten years, it started evolving into more virulent strains, which now trigger more severe cancers and afflict chickens at earlier ages.

Andrew Read from Pennsylvania State University thinks that the vaccines were responsible. The Marek’s vaccine is “imperfect” or “leaky.” That is, it protects ...

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