The purpose of this clinical study is to evaluate the immunogenicity and immunopersistence of the ACYW135 meningococcal polysaccharide conjugate vaccine (CRM197 vector).The protocol consists of two parts: Part 1 enrolled 660 eligible participants aged 6 to 23 months, which has now been completed. Part 2 : Open Clinical Researchplans to enroll approximately 100 eligible participants from Part 1 who have completed immunopreservance blood sampling. Participants will receive a single booster dose of the ACYW135 group meningococcal polysaccharide conjugate vaccine (CRM197 vector) at age 3 years (but not yet 4 years).

Meningococci are divided into 12 serogroups, and 95% of meningococcal cases are caused by serogroups A, B, C, X, W, and Y. Meningococcal epidemic flora can change, and the reasons are related to various factors such as meningococcal strain variation, human mobility transmission, and vaccination against different serogroups of meningococci, etc. The emergence of new serogroups of meningococci can cause the original vaccine to lose its immune protective efficacy and require a new vaccine immunization prevention strategy. the ACYW135 group meningococcal polysaccharide conjugate vaccine was approved in 2016 for a whole population The vaccine has been approved by the State Drug Administration for the prevention of epidemic meningoencephalitis caused by meningococci of groups A, C, W135 and Y in children from 3 months to 3 years of age (47 months of age).

Part Ⅰ

Inclusion Criteria:

• Children aged 6 to 23 months at the time of screening
• Volunteers who have not received any vaccine containing a epidemic encephalitis component or who have received only the group A polysaccharide flu vaccine and have been vaccinated more than 6 months since their last vaccination. 6 months or more between the last vaccination and the last vaccination
• The legal guardian or delegate has given informed consent, voluntarily signed the informed consent form, and is able to comply with the requirements of the clinical study protocol

Exclusion Criteria:

• Fever before inoculation, axillary temperature >37.0℃
• History of epilepsy, convulsions or seizures or history of psychiatric disorders or family history
• During an acute illness episode; with a serious chronic illness or a condition that is in a progressive stage that cannot be smoothly controlled (or an acute episode)
• Known allergy to a component of the vaccine, especially to diphtheria toxoid
• Live attenuated vaccine received within 14 days or other vaccine received within 7 days prior to enrollment
• Other circumstances that, in the judgment of the investigator, make participation in this clinical study inappropriate
• Those who had a severe allergic reaction after the previous dose of vaccine
• Those with serious adverse reactions causally related to the previous dose of vaccination
• Newly identified or newly occurred cases after the first vaccination that do not meet the inclusion criteria for the first dose or meet the exclusion criteria for the first dose will be determined by the investigator whether to continue to participate in the study

Part Ⅱ

Inclusion Criteria:

• The first phase of the study has been completed, involving the (0, 3) month vaccination schedule group for subjects aged 6 to 11 months. All subjects received two doses as required, and blood samples were collected 30 days after the primary vaccination series.
• Children aged 3 years at the time of screening
• The trial participant is able and willing to comply with the requirements of the clinical trial protocol and can complete the 1-month study follow-up.
• The trial participants did not receive any meningococcal vaccine booster doses at age 3 years
• The legal guardian or authorized representative voluntarily signs the informed consent form with full knowledge and understanding, and is able to comply with the requirements of the clinical research protocol.

Exclusion Criteria:

• Fever prior to vaccination, with axillary temperature >37.0°C
• History of epilepsy, convulsions, or seizures; psychiatric history or family history of psychiatric disorders
• During acute phase of illness; severe chronic disease or progressive condition not well controlled (or in acute phase)
• Known allergy to any vaccine component, particularly diphtheria toxoid
• Receipt of live attenuated vaccines within 14 days or other vaccines within 7 days prior to enrollment
• Other conditions deemed ineligible for this clinical study by the investigator