In Mexico City, where schools and businesses were shuttered during the first H1N1 outbreak last spring, the initial fear surrounding the virus has lifted, but health officials warn that young children are being unexpectedly hard hit this flu season.

Distribution centers around the country began receiving shipments of the much anticipated H1N1 flu vaccine this week. Those costs, as well as the economic blow of closed schools and lost productivity, could set back the fragile U.S. economy.