Data analysis
Data analysis was performed using Excel 2021 and statistical software SPSS 26. Symptoms were presented as percentages of the group total. Continuous variables were expressed as medians (interquartile ranges (IQR)) and analyzed by the independent samples Mann–Whitney U test. Categorical variables were analyzed by the chi-square test, withP < 0.05 considered statistically significant. We assessed the VEs against SARS-CoV-2 Omicron BA.2 variant infection of partial vaccinations, full vaccinations, and booster vaccinations compared with no vaccination, using binary logistic regression models to estimate the odds ratio (OR) of vaccination among case and control groups, adjusting by prespecified covariables considered to be potential confounders: gender, age, and time since the last dose of vaccination, expressed by OR and 95% confidence interval (95% CI). Furthermore, we performed subgroup analyses on gender, age groups, vaccine type, time since the last dose of vaccination (months), and type of booster vaccination.
The unadjusted OR was only used to calculate the crude VE, and the adjusted VE was estimated from the adjusted OR. The following formula was used: crude VE =(1 − crude OR)×100%; adjusted VE = (1 – adjusted OR) × 100%.