title: Reply to Koo et al.
authors.affiliation: Maternal and Pediatric Infectious Disease Branch, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland.
authors.forename: George K
authors.initials: GK
authors.lastname: Siberry
authors.affiliation: Center for Biostatistics in AIDS Research, Harvard T. H. Chan School of Public Health, Boston, Massachusetts.
authors.forename: Denise L
authors.initials: DL
authors.lastname: Jacobson