**3. Results**

### *3.1. General Demographic Characteristics of the Samples*

A total of 3136 serum samples collected from the residents of Yuexiu District in Guangzhou City (*n* = 699), Liwan District in Guangzhou City (*n* = 1386), Torch Development Area in Zhongshan City (*n* = 180), and Xiaolan Town in Zhongshan City (*n* = 871) were selected via stratified cluster sampling rooted in the database for serological testing. The study population had a male: female ratio of 1:1.92. Age statistics showed that the age group of ≥66 years old accounted for the largest proportion of the study population (25.86%), followed by 51–65 years old.

Finally, 474 subjects, including 237 cases and 237 controls, were recruited successfully (Figure 1). No statistical difference in gender (*p* = 0.950) and age (*p* = 0.127) existed between persons who were willing to receive the questionnaire survey and those who were unwilling to receive the questionnaire survey. The gender ratio was 1:1.66 (male: female) in both the case group and the control group. The demographic characteristics of the two groups were comparable (Table 1).

**Figure 1.** Consort diagram of cases and controls used in the study.


**Table 1.** Demographic characteristics of cases and controls.

a Except where otherwise indicated, values are the number (percentage) of patients with the characteristic. b Floating population refers to migrants who live locally for 6 months or less. c The minimum wage in Guangzhou City is 2100 yuan per month and the minimum wage in Zhongshan City is 1720 yuan per month.
