1. Institute of Global Health and Infectious Diseases, The University of North Carolina at Chapel Hill , Chapel Hill, North Carolina , USA
2. Department of Biostatistics, Gillings School of Public Health, The University of North Carolina at Chapel Hill , Chapel Hill, North Carolina , USA
3. Section of Infectious Disease, Department of Pediatrics, Tulane University School of Medicine , New Orleans, Louisiana , USA
4. John F. Kennedy Memorial Medical Center , Monrovia , Liberia