Laboratory tests
CYTOPATHIC EFFECTS, HEAT, DISEASE, VIRAL DISEASE
NAb titers in the sera of participants were detected by using the live-virus microneutralization assay based on inhibition of cytopathic effects (CPE). A total of six SARS-CoV-2 live viruses, including Omicron BA.1.1 (NPRC 2.192100005), Omicron BA.2 (NPRC 2.192100010), Omicron BA.4 (NPRC 2.192100012), Omicron BA.5 (NPRC 2.192100015), prototype (QD-01), Beta (GD84) and Delta (GD96) strains, were tested. These viruses were provided by the National Institute for Viral Disease Control and Prevention, the Chinese Center for Disease Control and Prevention (China CDC), Beijing, China. The live-virus microneutralization assays were also performed in the Biosafety Level-3 Laboratory (BSL-3) of the National Institute for Viral Disease Control and Prevention, China CDC.In the CPE-based live-virus microneutralization assay, serum samples were firstly inactivated by heat at 56 °C for 30 min, followed by a series of twofold dilutions starting from 1:4 for pre-booster serum samples and 1:8 for post-booster samples. The diluted serum was mixed with an equal volume of SARS-CoV-2 solution, which contains 100 TCID