1. Center for Health Systems Research, National Institute of Public Health, Cuernavaca, Morelos 62100, Mexico
2. International Center for Equity in Health, Federal University of Pelotas, Pelotas 96020-220, Brazil
3. Department of Global Health, Boston University School of Public Health, Boston, MA 02118, USA
4. National School of Nursing and Obstetrics, National Autonomous University of Mexico, Mexico City 14370, Mexico