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段落1

This is Scientific American 60-Second Science. I'm Christopher Intagliata. Got a minute?

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In the 1960s a cancer-causing herpes virus was ravaging the poultry industry.

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The virus caused what's called Marek's disease—and killed 1 to 2 percent of the birds.

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"Given that there are billions of birds in the industry, that's a lot of birds."

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Andrew Read, an evolutionary biologist at Penn State University.

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He says the virus was easy to catch.

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"So the dander of chickens if full of the virus. If you shake a chicken, the virus drops out."

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Then, in 1970 a new vaccine put an end to most of the deaths.

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But the poultry vaccine, unlike most, was a so-called "leaky" or "imperfect" vaccine.

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"The vaccine is lifesaving, but it allows the infection to persist and transmit from the host."

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Meaning you could still shake a vaccinated chicken¡ªand make it rain viruses.

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Now, Read and his colleagues have shown that these leaky vaccines may actually give some viral strains an evolutionary leg up.

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Because the most virulent strains usually wipe out unvaccinated birds in just 10 days, not enough time for the birds to infect many others.

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The viruses are essentially so "hot" they burn themselves out.

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But vaccinated birds survive infection with the hot virus, and shed it for weeks, allowing strains that would otherwise die out to stick around, and kill any unprotected birds.

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The study appears in the journal PLoS Biology.

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Of course, none of this is reason to doubt the efficacy of vaccines.

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"Vaccines have been one of the most important public health interventions.

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And the most cost effective we've ever had.

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And they're critically important in food chain security as well.

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So vaccines themselves are fantastic."

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But several imperfect vaccines for malaria are currently being tested.

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If they're approved, he says, we'll need to use other measures, like bed nets, to block transmission, remembering that not all vaccines are a one-shot deal.

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Thanks for the minute, for Scientific American 60-Second Science. I'm Christopher Intagliata.

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