The Market for Flu Shots with Spillover Benefits (A Positive Externality). The market demand curve does not reflect the positive externality of flu vaccinations, so only QMarket will be exchanged. This outcome is inefficient because the marginal social benefit exceeds the marginal social cost. If the government provides a subsidy to consumers of flu shots, equal to the marginal social benefit minus the marginal private benefit, the level of vaccinations can increase to the socially optimal quantity of QSocial