P OR 95% C.I. Lower Upper Gender female vs. male 0.001 4.52 1.81 11.28 Age ≤30 vs. >30 0.052 2.72 0.99 7.46 Marital status Married vs. single 0.001 3.96 1.71 9.17 Nation Han vs. Minority 0.122 5.21 0.64 42.12 Resident place local vs. transient 0.418 0.67 0.26 1.75 Education ≤9 years vs. >9 years 0.005 3.12 1.41 6.89 Monthly income 601-2000 vs. ≤600 0.420 1.82 0.42 7.86 >2000 vs. ≤600 0.208 2.61 0.59 11.58 missing1 vs. ≤600 0.155 2.93 0.67 12.93 IDU yes vs. no 0.033 2.81 1.09 7.24 Commercial sex in last 12 months yes vs. no 0.943 1.04 0.40 2.66 STD symptoms in last 12 months yes vs. no 0.219 1.73 0.72 4.15

Table 2: Logistic regression analysis of sample characteristics associated with syphilis in Guangdong Dongguan (syphilis prevalence 9.6%)