Statistical analysis
Data were analyzed with SPSS Statistics 19 (SPSS, Inc., USA). Distribution normality was checked for all variables including age and gender. The comparison of the age was analyzed by Mann-Whitney U test. Due to the unequal group sizes and the data variance, Chi-square test or Fisher‘s exact test was used to analyze the association between age, gender and year groups with respiratory viruses. A P value of <0.05 was considered statistically significant.