We hypothesized that sexual behavior and risk of STDs infection in the elderly population would be different depending on congregating places. As such, congregating places for the elderlies were selected on the basis of risk of STDs infection. Senior welfare centers were selected as places where risk of STDs infection is expected to be low, and urban public parks were selected as places where risk of STDs infection is expected be high. To find out the nationwide status, multiple places were selected around the country. For the study of the elderlies with low risk of STDs, senior welfare center in the city of Incheon and Ulsan were selected, and for the study of the elderlies with high risk of STDs, Jongmyo Park in the city of Seoul and Dalseong Park in the city of Daegu were selected. Visits were made between July 2015 and December 2015. Informed consent was received from all elderlies who volunteered for the study. They were first asked to fill out a survey on sexual behavior and STDs awareness, developed for the purpose of the current study (Supplementary material). Then, first-void urine sample was obtained for urine polymerase chain reaction (PCR) test (Seegene Medical Foundation, Seoul, Korea) to test for Neisseria gonorrhoeae, Chlamydia trachomatis, Mycoplasma genitalium and Trichomonas vaginalis. The survey contained components on socioeconomic status, sexual behavior, awareness regarding STDs, STDs status and sexual health education. The results of the survey and urine PCR test of the two groups were compared.

HIRA database contains all health insurance claims issued by health institutions in South Korea. Since all Koreans are required to enroll in the National Health Insurance Service by law, the HIRA database is a representative data of the whole country. Data on insurance claims with the disease codes containing N. gonorrhoeae and C. trachomatis between 2010 and 2016, was analyzed. The actual number and proportion of patients over 65 years of age were investigated and the annual trend was analyzed.

Table 1 describes the baseline demographics and socioeconomic status of the elderlies who completed the survey. There were a total of 337 participants aged over 60, 204 in the low-risk group and 133 in the high-risk group. The distribution of age group was similar between the two groups. The distribution of sex was different; male to female ratio was 47.5:52.5 in the low-risk group, whereas in the high-risk group, it was 88.0:12.0, showing male predominance in the high-risk group. The education level and income level was generally higher in the low-risk group.

The rest of the survey components are described in the supplementary tables. Supplementary Tables 1 to 7 describe questions regarding sexual behavior, Supplementary Tables 8 to 11 describe questions regarding STDs awareness and Supplementary Tables 12 to 14 describe questions regarding sexual health education.

Supplementary Table 1 describes the frequency of sexual intercourse. The overall frequency of sexual intercourse did not show significant difference between the two groups. Respondents who had sexual intercourse over 1 to 2 times per 3 months was 36% and 32% in the low-risk and high-risk group, respectively. However when stratified by sex, 54% of males maintained sexual life whereas the number was 31% for females, implying that males play a more dominant role in sexual life in the elderly population. Supplementary Table 2 describes the number of sexual partners. In the low-risk group, 97.5% of the respondents had 1 or no sexual partners and only 0.5% had 3 or more partners. In the high-risk group, 94% of the respondents had 1 or no sexual partners and 5% had 3 or more partners. The percentage of respondents who had multiple partners was 3 times higher in the high-risk group. Also, respondents who had multiple partners were all males. Supplementary Table 3 describes the types of sexual partners. In the low-risk group, over 90% were spouse and only 1% was prostitutes. In the high-risk group, 70% were spouse and 7% was prostitutes, showing a significantly higher percentage of prostitution. Supplementary Table 4 describes condom use during sexual intercourse. The percentage of condom use was 3% and 14% in the low-risk and high-risk group, respectively. Only 1% in the low-risk group and 10% in the high-risk-risk group responded that they used condoms almost always. This result shows that condom use is very low among sexually active elderly population. Supplementary Table 5 describes presence of erectile dysfunction among male respondents. The percentage of moderate to severe erectile dysfunction was 77% and 80% in the low-risk and high-risk group, respectively. This result shows that majority of males in both groups suffered from some degree of erectile dysfunction. Supplementary Table 6 describes use of phosphodiesterase-5 inhibitors. The percentage of respondents that used phosphodiesterase-5 inhibitors was 24% and 28% in the low-risk and high-risk group, respectively, showing a similar percentage of use for both groups. Supplementary Table 7 describes contribution of phosphodiesterase-5 inhibitors to sexual life. The percentage of respondents who thought phosphodiesterase-5 inhibitors helped their sexual life was 31% and 30% in the low-risk and high-risk group respectively, showing similar results.

Supplementary Table 8 describes history of STDs infection and time of the infection. The percentage of respondents with history of previous STDs infection was 17% and 27% in the low-risk and high-risk group, respectively. In the low-risk group, none of the past STDs infection occurred within 10 years, whereas in the high-risk group, 9% of the STDs infection occurred within 10 years. Supplementary Table 9 describes type of STDs. In the low-risk group, urethritis was 6%, gonorrhea 66%, syphilis 6% and genital herpes 6%. In the high-risk group, urethritis was 17%, gonorrhea 75% and genital herpes 3%. Gonorrhea was the predominant STD for both groups. Supplementary Table 10 describes type of sexual partner from whom STDs was infected. In both groups, prostitute was the most common sexual partner from whom STDs was infected. It constituted 53% in the low-risk group and 55% in the high-risk group, without showing significant difference. Supplementary Table 11 describes places where STDs were treated. In both groups, hospitals were the most common place respondents received treatment for STDs. 86% of the respondents for both groups received treatment in hospitals.

Supplementary Table 12 describes experience with and intention to receive sexual health education. Approximately 47% in the low-risk group and 41% in the high-risk group answered they were willing to receive sexual health education, showing similar response. However, past experience with sexual health education showed significant difference. Approximately 29% in the low-risk group answered they had experience with sexual health education, whereas only 10% in the high-risk group experienced sexual health education. This implies that elderlies congregating in senior welfare centers had more opportunities than elderlies congregating in public parks in receiving sexual health education. Supplementary Table 13 describes preferred institution to receive sexual health education for low-risk group and high-risk group, respectively. In the low-risk group 39% answered they preferred senior welfare centers, and 18% answered public health centers. In the high-risk group, 51% answered they preferred public health centers, 16% answered senior welfare centers and 4% answered counseling centers. This difference is thought to be due to convenience in approachability depending on their congregating places. Supplementary Table 14 describes preferred person to receive sexual health education. In the low-risk group, 34% answered they preferred doctor/nurse and 34% preferred counselor. In the high-risk group, 56% answered they preferred doctor/nurse and 27% preferred counselor.

Table 2 describes the urine PCR results of the elderlies who completed the survey. There was one case of C. trachomatis in the low-risk group and one case of M. genitalium in the high-risk group.

Table 3 describes the actual number and percentage of N. gonorrhoeae and of C. trachomatis infection in population over 65 years of age between 2010 and 2016 from the HIRA database. The overall percentage of N. gonorrhoeae infection in population over 65 years of age did not change much between 2010 and 2016. It was 1.8% in 2010 and 1.9% in 2016. When stratified by sex, the percentage of male ranged between 1.3% to 1.7% and the percentage of female ranged between 3.0% to 4.8%. The percentage of C. trachomatis infection in population over 65 years of age steadily declined. It was 1.6% in 2010 and 0.9% in 2016. When stratified by sex, the percentage of male declined from 2.7% in 2010 to 1.3% in 2016, whereas the percentage of female was stable around 0.8%. Fig. 1 graphically describes this trend.