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https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4516275/ Here's a study. https://www.nationalgeographic.com/science/phenomena/2015/07/27/leaky-vaccines-enhance-spread-of-deadlier-chicken-viruses/ Here's a National Geographic article. And here's a wikipedia page that discusses it in some detail...

http://www.poultryimprovement.org "The National Poultry Improvement Plan was established in the early 1930's... to eliminate Pullorum Disease caused by Salmonella pullorum which was rampant in poultry and could cause upwards of 80% mortality in baby poultry. The program was later extended and...