[link to www.pbs.org (secure)]


cliff notes from the article:

"A leaky vaccine is one that keeps a microbe from doing serious harm to its host, but doesn’t stop the disease from replicating and spreading to another individual.

The scientists now believe that this vaccine has helped this chicken virus become uniquely virulent.

The hottest strains killed every unvaccinated bird within 10 days, and the team noticed that barely any virus was shed from the feathers of the chickens during that time. (The virus spreads via contaminated dust in chicken coops). In contrast, vaccination extended the lifespan of birds exposed to the hottest strains, with 80 percent living longer than two months. But the vaccinated chickens were transmitting the virus, shedding 10,000 times more virus than an unvaccinated bird.

“Previously, a hot strain was so nasty, it wiped itself out. Now, you keep its host alive with a vaccine, then it can transmit and spread in the world,” Read said. “So it’s got an evolutionary future, which it didn’t have before.”"