Statistical analysis
Categorical variables were described as frequency rates and percentages, and continuous variables were described using mean and standard deviation values. Proportions for categorical variables were compared using the χ2 test. The Fisher exact test was used when the data were limited. One way analysis of variance (ANOVA) was used to investigate the association between the mean age of staff with their IgG results. All statistical analyses were performed using SPSS (Statistical Package for the Social Sciences) version 20.0 software (SPSS Inc). A comparison with p less than 0.05 was considered statistically significant.