This study is a randomized and controlled phase Ⅳ clinical trial in children aged 3-5 years old. The purpose of this study is to evaluate the immunogenicity and safety of COVID-19 vaccine (Vero cell), Inactivated co-administration with EV71 vaccine.The COVID-19 vaccine was manufactured by Sinovac Research & Development Co., Ltd, and the EV71 vaccine was manufactured by Sinovac Biotech Co.A total of 520 subjects aged 3-5 years old will be enrolled.Subjects will be randomly divided into 2 groups in a ratio of 1:1.The experimental group is the combined immunization group, which will receive the first dose of COVID-19 vaccine and the second dose of COVID-19 vaccine and EV71 vaccine on day 28;Control group is the Non-combined immunization group , which will receive the first dose of COVID-19 vaccine on day 0 ，the first dose of EV71 vaccine on day 14 ，the second dose of COVID-19 vaccine on day 28 and the second dose of EV71 vaccine on day 42.
This study is a randomized and controlled phase Ⅳ clinical trial of the COVID-19 vaccine (Vero cell), Inactivated manufactured by Sinovac Research & Development Co., Ltd. The purpose of this study is to evaluate the immunogenicity and safety of COVID-19 vaccine (Vero cell), Inactivated co-administration with EV71 vaccine