I heard a podcast interview with a small-town doctor - maybe This American Life? She said that they lose money on vaccines, because they’re expensive, and to buy enough to keep a stock on hand, the vaccines end up expiring before they get used. Or the flu season ends and the vaccines are of no use since a different flu will be around the next year. So the higher insurance reimbursements are to help compensate them for continuing to provide a product at a loss.