Statistical analysis
REGRESSION
The level of antibodies was presented as the geometric mean titer (GMT) and geometric mean fold increase (GMFI). The 95% confidence interval (CI) was calculated on the basis of the t-distribution of the log-transformed values back-transformed to the original scale. The Student’s t-test, Chi-squared test, or Fisher’s exact test was applied to analyze categorical data. A Cox model was used to estimate the cumulative probability of breakthrough infection. A multivariable logistic regression analysis was performed to test the adjusted association between the main independent variable, which was defined as the type (inactivated vaccine or Ad5-nCoV) of the fourth-dose vaccine, and 1) the seropositivity rates of anti-RBD IgG and neutralizing antibodies against the BA.4/5 pseudovirus and 2) the risk of breakthrough infection with SARS-CoV-2. 