[1] Univ North Carolina Chapel Hill, UNC Project, Lilongwe, Malawi
[2] Univ North Carolina Chapel Hill, Sch Med, Chapel Hill, NC USA
[3] Univ North Carolina Chapel Hill, Sch Publ Hlth, Dept Epidemiol, Chapel Hill, NC USA
[4] Univ North Carolina Chapel Hill, Sch Publ Hlth, Dept Hlth Behav, Chapel Hill, NC USA
[5] Univ Southampton, Fac Med, Southampton, Hants, England
[6] Univ Southampton, Fac Social Human & Math Sci, Southampton, Hants, England
[7] Ohio State Univ, Dept Epidemiol, Columbus, OH 43210 USA