Can leaky vaccines favor nastier viruses?

Imperfect Marek’s disease vaccination keeps infected chickens alive longer, boosting transmission of strains too lethal to persist without vaccines.

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How can imperfect vaccination enhance virulent MDV spread?

By keeping infected birds alive, extending transmission of otherwise too-lethal strains.

How lethal are hyperpathogenic MDV strains in unvaccinated birds?

Mortality rates up to 100%.

What happened to sentinel infection timing with vaccination?

More rapid infection; median positivity 9 days earlier.

What evolutionary idea was experimentally tested?

The imperfect-vaccine hypothesis for virulence evolution.

What does vaccination do to infectious periods of hyperpathogens?

Greatly prolongs them (hens or offspring vaccinated).

What do authors say vaccination is sufficient to do today?

Maintain hyperpathogenic strains in poultry flocks.

Why is natural selection against hyperpathogens relaxed?

Vaccination enhances host survival.

How much did vaccination prolong infectious periods in sentinels?

About 2 weeks.

Did authors claim vaccines alone caused historical MDV evolution?

No—they do not demonstrate vaccines caused the original evolution.

What disease features defined early vs modern MDV?

Earlier mild paralysis vs modern lymphomas and high mortality.

What is the key fitness benefit vaccination gives hypervirulent strains?

Hosts survive long enough to shed and transmit.