Department of Epidemiology and Biostatistics, University of South Carolina, Columbia, SC 29201, USA.|Department of Mathematics, University of Arizona, Tucson, AZ 85721, USA.|Department of Mathematics, University of Arizona, Tucson, AZ 85721, USA.|Department of Applied Mathematics & Statistics, Johns Hopkins University, Baltimore, MD 21218, USA.|Department of Applied Mathematics & Statistics, Johns Hopkins University, Baltimore, MD 21218, USA.|Department of Biostatistics, Yale School of Public Health, New Haven, CT 06520, USA.