To enhance quality assurance of vaccine distribution by public health programs in the US, various methods for packing vaccines were validated. Validation involved both tests in an environmental chamber and actual shipping of packages by commercial overnight delivery service. Dry ice was used with vaccines needing to be kept at temperatures lower than −14°C, and water-based cold packs with other vaccines. The latter could be used in two ways. When frozen, and placed over two or three faces of well-insulated boxes, assortments of vaccines were kept cold but not frozen for 2 days or more. However, packages with −15°C cold packs may reach