Spss statistical software package release 23

Descriptive statistics and univariate analyses were performed. The outcome variable was the anti-S-RBD titer at least 3 weeks after the boost dose. Differences in anti-S-RBD levels and PROT-80 according to demographic and clinical parameters were evaluated. Categorical variables were expressed as numbers (percentage) and analyzed using the Chi-square test or Fisher’s exact test. Student’s t-test and the Mann–Whitney U test were used to compare independent continuous variables, expressed as median (interquartile range, IQR). For related continuous variables, the Wilcoxon signed-rank test was applied. Factors associated with anti-S-RBD levels with a p < 0.2 in the univariate analysis were evaluated in a general linear model adjusted for age and sex. Likewise, multivariate logistic regression models were developed to identify factors associated with PROT-80. Adjusted odds ratios (AOR) with their corresponding 95% confidence intervals (CI95) were calculated. Statistical analyses were carried out using the SPSS statistical software package release 23.0 (IBM SPSS Inc., Chicago, IL, USA).