[1] JOHNS HOPKINS UNIV, SCH HYG & PUBL HLTH, BALTIMORE, MD 21218 USA
[2] VET ADM MED CTR, BALTIMORE, MD 21218 USA
[3] US PHS HOSP, DEPT INTERNAL MED, BALTIMORE, MD 21211 USA
[4] UNIV MARYLAND, SCH MED, DIV GASTROENTEROL, BALTIMORE, MD 21201 USA
[5] US FDA, BUR BIOL, DIV BLOOD & BLOOD PROD, BETHESDA, MD 20014 USA