[1] Tufts New England Med Ctr, Div Pediat Infect Dis, Boston, MA 02111 USA
[2] Univ Maryland, Dept Pediat, Baltimore, MD 21201 USA
[3] St Christophers Hosp Children, Infect Dis Sect, Philadelphia, PA 19134 USA
[4] Ctr Dis Control & Prevent, Natl Immunizat Program, Atlanta, GA 30333 USA