Three decades of USAID investments in immunization through the child survival revolution.

The US Agency for International Development (USAID) launched a significant effort to improve global immunization coverage in the mid-1980s, beginning a long history of investments in various approaches to supporting the improvement of national vaccination programs in developing countries. As Primary Health Care evolved, USAID's approach to immunization also evolved, heavily influenced by the child survival revolution, a period when the global community struggled to define an approach that incorporated the essence of the Alma-Ata Conference with the selective primary health-care approach. Eventually, what became known as the 'twin engines' approach, a focus on two high impact interventions-immunization and oral rehydration therapy-would characterize USAID's child survival program.