The idea is textbook economics. People respond to incentives. And incentives can be used not just for the sake of individuals, but for the benefit of society as a whole. Like insurance discounts for safe driving. Or cash payouts for employees who quit smoking. And they have been tried with vaccinations — for example in rural India where childhood immunization rates were low and even a relatively small incentive — a sack of lentils and metal plates — raised them significantly.