In many industries, multi-product sellers offer bundled loyalty discounts to buyers who refrain from purchasing competing products from rivals. These contracts can increase sales volumes by helping sellers establish a strong customer base across their product portfolios, but they also risk foreclosing competing sellers, even those offering superior products. I examine the welfare implications of bundled loyalty discounts in the context of physician-administered vaccines, which medical providers purchase and administer directly to their patients. I use prior antitrust cases involving vaccines to recover the proprietary terms of bundled discount contracts. I then develop a model of medical providers’ demand and pharmaceutical companies’ supply of vaccines. Using all-payer claims data from Colorado to estimate the model, I recover medical providers’ preferences, vaccines’ marginal costs, and the magnitude of bundled discounts that rationalizes providers’ observed purchasing patterns. I use the model to evaluate the equilibrium and welfare effects of banning bundled discounts, allowing sellers and buyers to re-optimize their choices. I show that, in the absence of discounts, providers purchase fewer and lower-quality products, particularly at the expense of optional vaccines not required for school access. Insurers benefit in the short term through lower immunization coverage costs. The effect on pharmaceutical companies varies, depending on the scope and composition of their vaccine portfolios.