1. Viral Epidemiology and Immunity Unit, Laboratory of Infectious Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892-3203, USA.
2. Sustainable Sciences Institute, Managua 14007, Nicaragua.
3. Department of Epidemiology, School of Public Health, University of Michigan, Ann Arbor, MI 48109-2029, USA.
4. Centro de Salud Sócrates Flores Vivas, Ministry of Health, Managua 12014, Nicaragua.
5. Laboratorio Nacional de Virología, Centro Nacional de Diagnóstico y Referencia, Ministry of Health, Managua 16064, Nicaragua.
6. Division of Infectious Diseases and Vaccinology, School of Public Health, University of California, Berkeley, Berkeley, CA 94720-3370, USA.