Losing momentum : are childhood vaccine supplies adequate? : hearing before the Subcommittee on Public Health of the Committee on Health, Education, Labor, and Pensions, United States Senate, One Hundred Seventh Congress, second session, on examining the adequacy of childhood vaccine supplies, focusing on the extent that recent shortages have affected immunization policy and programs, what factors have contributed to recent shortages, and what strategies are federal agencies considering to help mitigate the disruptions in supply, September 17, 2002.