[1] Univ Washington, Dept Pediat, Seattle, WA 98195 USA
[2] Grp Hlth Cooperat Puget Sound, Ctr Hlth Studies, Seattle, WA 98121 USA
[3] Univ Arizona, Resp Sci Ctr, Tucson, AZ USA
[4] Harvard Univ, Sch Med, Boston, MA USA
[5] Univ Adelaide, Dept Med, Woodville, SA, Australia
[6] Brigham & Womens Hosp, Channing Lab, Dept Med, Boston, MA 02115 USA