ABSTRACT
A practical formulation that has been a commercial product since the mid-1980s consists of lyophilized vaccine in enteric-coated, acid-resistant, capsules (1,2). In a randomized, placebo-controlled field trial in Santiago, Chile, three doses of this enteric-coated formulation given within one week provided 67% efficacy during the first three years of follow-up (2) and 62% protection over seven years of follow-up (35). Four doses of Ty21a in enteric-coated capsules given within eight days are significantly more protective than two or three doses (33). When the enteric-coated capsule formulation of Ty21a was licensed in the United States by the Food and Drug Administration in late 1989, it was with a recommended schedule of four doses given at an every other day interval; other countries use a three-dose immunization schedule.
