MMP9 and susceptibility to pneumonia measurement: Rs17577 was reported to be associated with pediatric asthma [49, 50], breast cancer [51], and ischemic stroke [52] etc. Although the P-value (P = 0.0198) of rs17577 SNP between control and pneumonia group did not meet the threshold (0.00083) of Bonferroni’s correction, this SNP may have important impact on MMP9 expression and potentially was a risk factor of severe pneumonia.