banjo,
One dose *may* protect many of them - but the manufacturer didn't put that 'booster needed' phrase in the label just to sell more vaccine - the trials they conducted to satisfy USDA requirements for licensure showed that the booster, administered as stated, was necessary to provide a specific level of protection ( like, maybe, 95% resisted challenge or demonstrated detectable antibody levels).
Millions probably survive with one dose - but is it because they were adequately immunized, or did they just not have an exposure? How do you know? It's cheap. Give the booster. Yeah, I know it's a PITA getting 'em up and running 'em through the alley/chute again, but... I'm just sayin'...