Bridging Clinical Trial of EV71 Vaccine in Vietnamese Children

The purpose of this study is to evaluate the immunogenicity and safety of an inactivated Enterovirus Type 71 (EV71) vaccine, manufactured by Sinovac, in healthy Vietnamese children aged 6 to 71 months. Hand, Foot, and Mouth Disease (HFMD) is a common infectious disease in children, and EV71 is a major cause of severe complications and outbreaks in the Asia-Pacific region, including Vietnam. While this vaccine has demonstrated high efficacy and a favorable safety profile in previous clinical trials conducted in China, this study aims to generate specific bridging data for the Vietnamese pediatric population. This is a single-armed bridging clinical trial that plans to enroll approximately 320 healthy participants. Each participant will receive two doses of the EV71 vaccine administered intramuscularly on a 0 and 30-day schedule. The primary objective is to assess the seroconversion rate of neutralizing antibodies 30 days after the second dose. Safety will be assessed by monitoring the incidence of adverse reactions and serious adverse events (SAEs) throughout the study duration (approximately 2 months).

Inclusion Criteria: * Healthy children aged 6\~71 months when receiving the first dose of investigational EV71 vaccine; * Participants' legal guardians are able to understand and sign the informed consent voluntarily; * Participants are able to comply with the study procedures based on the assessment of the investigator; * Participants should provide verifiable identification, be reachable by the investigator and must be able to contact the investigator throughout the study period. Exclusion Criteria: * Prior history of hand, foot and mouth disease (HFMD); * Have been exposed to a case with laboratory-confirmed (virological diagnosis, e.g., via PCR or virus isolation) HFMD at home, day care, or other settings within 2 weeks before enrollment; * Prior participated in clinical trial of EV71 vaccine and received any investigational EV71 vaccine; * Known allergy to vaccines or vaccine ingredients, or serious adverse reactions to vaccines, such as urticaria, dyspnea, angioneurotic edema; * Any known or history of autoimmune diseases (including but not limited to systemic lupus erythematosus, rheumatoid arthritis, etc.) or immunodeficiency/immunosuppression (such as HIV infection, organ transplantation), asplenia; * Received immunosuppressive for ≥14 days (e.g., prednisone equivalent of ≥2 mg/kg), or cytotoxic drug before enrollment, or plans to receive these treatments during the study participation. * Receipt of blood products or immunoglobulins within the past 3 months before enrollment in this study, or scheduled to receive these treatments during the study period; * Receipt of other investigational vaccines within 30 days prior to vaccination in this study; * Receipt of other vaccines within 14 days prior to vaccination in this study; * Presence of any acute disease (moderate to severe illness, with or without fever) or an acute exacerbation of a chronic condition within the 7 days prior to vaccination in this study. * Has fever on the day of vaccination, with the axillary temperature ≥ 37.5 celsius before vaccination; * Is participating in or planning to participate in other vaccine or drug clinical trials; * According to the investigator's judgment, the participant has any other factors that might interfere with the clinical trial results or pose additional risk to the participant due to participation in the study.