Efficacy trial of Vi polysaccharide vaccine against typhoid fever in south-western China

Objective To test the efficacy of locally produced Vi vaccine over a time p eriod of longer than one year. Methods A double-blinded, randomized field trial was performed in Guangxi Z huang Autonomous Region in south-western China, using 30 mug doses of local ly produced Vi. Enrolled subjects were 3-50 years of age, although the majo rity (92%) were school-aged children, who have the highest rate of typhoid fever in this setting. A total of 131271 people were systematically allocat ed a single dose of 30 mug of Vi polysaccharide or saline placebo. The stud y population was followed for 19 months, with passive surveillance conducte d in the Ministry of Health and the Regional Health and Anti-epidemic Centr e (HAEC). Clinically suspected cases of typhoid fever were confirmed by blo od culture, or by serological reaction with O-antigen (Widal tests). Findings After 19 months, there were 23 culture-confirmed cases of typhoid fever in the placebo group versus 7 cases in the Vi group (Protective effic acy (PE) = 69%; 95%;, CI = 28%, 87%). Most of the isolates were from school -aged children. 22 cases in the placebo group versus 6 in the Vi group (PE = 72%; 95% Cl = 32%, 82%). No serious post-injection reactions were observe d. The locally produced Vi polysaccharide vaccine showed levels of protecti ve efficacy similar to those for Vi vaccine produced in industrial countrie s. Conclusion The slightly higher dose of vaccine did not seem to alter effica cy significantly in China.