Objective To evaluate the safety and immunogenicity of Shigella flexneri 2a conjugate vaccine.Methods A random and double blind study were carried out to compare the safety sero conversion rates and geometric mean titer(GMT)increase of Shigella flexneri 2a conjugate vaccine with phosphate-buffered saline used as control.Results The results showed that no severe systemic and local reaction rates occurred in trial group,which were not statistically significant compared with the control group.The seroconversion rate of both two weeks and twelve weeks after two doses(increased by 4 folds) were 86.27% and 79.74%;for GMT were 1∶361.83 and 1∶326.21 and increased averagely 1.08 and 0.98 times,which showed significant difference with those of the control group.Conclusion Shigella flexneri 2a conjugate vaccine was safe and its immunogenicity was good in group over 2 years old.Trialregistration National food drugs surveillance administrative bureau,"Medicine Clinical Experiment Written directive from a superior"2003L03808 number.