The level of neutralizing antibody was detected by using the constructed pseudovirus covid-19 variant at baseline , 1 month , 6 months and 12 months after primary vaccination, within 15 days after booster immunization at 12 months . and the ability of IgG subclasses to neutralize and cross immunization of variant strains was analyzed.

In 150 individuals who received primary and booster COVID-19 inactivated vaccine, the neutralization ability of antibodies against covid-19 mutant after primary and booster immunization was evaluated by constructing pseudotyped SARS-CoV-2 variants. The contribution and influence of IgG subclasses on neutralization capacity were assessed by Elisa assay. All participants were recruited aged between 18 and 59 years old. Every participant was familiarized with the aim of the study and asked to sign an informed consent agreement and was required blood sampling at the day 0 of first dose vaccine and 1 month, 6 months after 2nd dose of primary immunization. All participants had received 1 dose of booster immunization at 12 months after fully primary vaccination. Then 150 people were divided into four groups, Blood samples were taken on days 0,3,7,10 and 14 to detect neutralizing antibodies and IgG subclasses separately. every sample was detected SARS-CoV-2 variants strain Alpha, Beta and Delta neutralizing antibody, and RBD-IgG1, IgG2, IgG3, and IgG4. every individual was conducted a detailed analysis correlation of isotype switching, neutralizing antibody titer and cross immunity against COVID-19 variants after primary and booster vaccination.

Inclusion Criteria:

• Between the ages of 18 and 59
• Healthy
• No vaccination with COVID-19 before become participant
• Agreed to take a blood sample

Exclusion Criteria:

• who cannot take the COVID-19 vaccine due to some personal reasons
• history of SARS-CoV, SARS-CoV-2, or Middle East respiratory syndrome infection
• high-risk epidemiology history within 14 days before enrolment (eg, travel or residence history in communities with case reports, or contact history with someone infected with SARS-CoV-2)
• axillary temperature of more than 37·0℃
• history of allergy to any vaccine component.

The healthy people aged between 18 and 59, and have not been infected with COVID-19, have not been vaccinated with COVID-19 vaccine, and agree to collect blood samples.