1. Department of Epidemiology and Biostatistics, Texas A&M University, College Station, TX, USA
2. Emory University School of Medicine, Atlanta, GA, USA
3. Virology Laboratory of the National Institute of Perinatology, Mexico City, Mexico
4. Department of Microbiology, National School of Biological Sciences of the National Polytechnic Institute, Mexico City, Mexico
5. Department of Epidemiology and Biostatistics, Temple University, Philadelphia, PA, USA