Introduction

Chlamydia is the most common notifiable disease in the European Union [1] and USA [2]. In 2014, over 1.4 million cases of chlamydia were reported to US Center for Disease Control and Prevention (CDC), the highest number of any infections ever reported [2]. It was estimated in 2008 that the lifetime direct medical cost of chlamydia had reached US$516.7 million (excluding indirect costs such as loss of productivity and other intangible costs) [3]. Up to 80% of women with genital chlamydia is asymptomatic, but itself can be both highly contagious and dangerous in terms of long-term health consequences such as pelvic inflammatory disease, infertility and ectopic pregnancy [4, 5] as well as increased risks of HIV infection [6]. All these complications can be avoided with early diagnosis and treatment.

Representative population-based prevalence data are the crucial first step in understanding the disease burden and its spread, creating the foundation for effective intervention, prevention and other control measures such as resource allocation, designing and delivery [7, 8]. These studies have been undertaken extensively in Europe [8–13], USA [14, 15], and China [16]. The National Surveys of Sexual Attitudes and Lifestyles (Natsal) from the UK has provided useful epidemiological data to inform guidelines in England [8], Scotland [17], and Wales [18]. Likewise, the US Preventive Services Task Force (USPSTF) has also recommended screening of all sexually active females below 25 years old since 2007 [19], again reinforced in 2014 [20].

With a population of 7.3 million people, Hong Kong is an important global city with frequent exchange with other parts of the world [21]. The epidemiological significance of this was evident in the Severe Acute Respiratory Syndrome (SARS) and avian flu outbreaks in recent history. It was estimated from a recent review and meta-analysis that the prevalence of travel-associated casual sex could be as high as 20%, about half were unsafe sex without condoms [22]. Amongst the 60 million annual visitors to Hong Kong, 77% of them were from mainland China [23]. Concurrently, sexually transmitted infections (STIs) have risen rapidly in mainland China; for examples, syphilis incidence has nearly tripled from 12.80 per 100,000 persons in 2006 to 31.85 per 100,000 in 2015 [24, 25]. Incidence caused by genital Chlamydia trachomatis (CT) has increased by nearly 40% from 37.20 per 100,000 persons in 2010 and 51.3 per 100,000 in 2014 in the neighbouring Guangdong Province [26, 27].

Hong Kong has never been included in any population-based STI prevalence studies and STIs are not notifiable locally. As a result, the limited data on STIs are either from key populations (e.g. female sex workers)[28] or specific settings such as government-run Social Hygiene Clinics (SHCs). Anecdotally an increasing trend of STIs was observed; for example, from 2011 to 2015, the STI cases diagnosed at SHCs had increased by 9.8% whilst primary and secondary cases of syphilis had been more than doubled [29, 30]. This study examined the prevalence of Chlamydia trachomatis infectionand risk factors among a population-based sample of 18-49-year-old residents in Hong Kong.

Methods

Results

Discussion

TeSSHS identifies CT prevalence among the general population in Hong Kong and maps it according to its geographical distribution. Contrast to the clinical findings from SHC, TeSSHS finds an overall low Chlamydia trachomatis prevalence in Hong Kong but has identified high prevalence in certain pockets of the population. A unique U-shape distribution of Chlamydia trachomatis infection in younger and older women was identified, which is very different from other population-based studies conducted elsewhere. It shows a hidden epidemic among the sexually active residents and finds travel history has significant roles to play in its transmission.

One main limitation is that TeSSHS has a relatively low response rate which is understandable given the sensitive nature of the study and is comparable to those reported in Natsal-3 [8]. One may also argue that perhaps those with higher perceived risks would be more forthcoming in participation. However, three indirect evidences may suggest otherwise. First, compared to the Chinese Health and Family Life Survey in 1999–2000, TeSSHS has a lower proportion of participants with ≥1 sex partners in the past 12 months i.e. 5.2%, (95% CI 3.3–8.2%) for males and 2.7% (95% CI 1.6–4.7%) for females compared to that of China study, 10.0% for males and 4.0% for females admitted so [16]. Secondly, compared to the Youth Sexuality Study (2011) in Hong Kong where 19.7% of male and 5.2% of female youth (18–27 years old) reported eposex partners in the past six months [34], the proportions of male and female aged 18–26 years old with ≥wisex partners in the past 12 months were 2.6% (95% CI 0.9–7.2%) and 3.3% (95% CI 1.1–9.7%) in TeSSHS respectively. Thirdly, the consistent condom use reported in the general population is 24% (95% CI 21.84–26.16%) [35] whilst the consistent condom use among TESSHS participants is 35.9%, indicating that they may be more health conscious and prepared to practise sexual risk reduction.

Another limitation was that no typing of PCR reactive samples was made in this project. Since most prevalence studies about chlamydial infections were carried out in Europe or USA, the ratio of distinct geno/sero types of Chlamydia trachomatis would be of high interest. Though the RT-PCR kit used could detect the great majority of CT strains, it could have missed some Chlamydia trachomatis strains that do not have any cryptic plasmid e.g. the new Swedish variant.

The overall prevalence of CT in Hong Kong i.e. 2.0% among female and 1.5% among male sexually experienced participants, is lower compared to those of China and global estimation but similar to those reported in European countries. In World Health Organization (WHO) global estimation of CT in 2012, the prevalence among 15–49 year old women is 4.2% (3.7–4.7%) [36] while in the Chinese Health and Family Life Survey, the prevalence equivalent is 2.6% (95% CI 1.6–4.1%) for females, and 2.1% (95% CI 1.3–3.3%) for males [16]. Findings in TESSHS is similar to the national prevalence study in France, e.g. CT prevalence is 1.6% (95% CI 1.0–2.5%) for women and 1.4% (95% CI 0.8–2.5%) for men between 18 and 44 years old in 2010 [11]. It is argued that the socioeconomic status and lifestyle of Hong Kong would be more comparable to that of developed countries [36].

This study also identifies that CT is more common among young individuals in Hong Kong, which is consistent with data from the USA [14], France [11], and the UK [8]. Among the sexually experienced, CT prevalence identified by TeSSHS is more comparable to the estimated prevalence among those aged 26 or below in the European countries with the pooled average prevalence at 3.6% (2.4–4.8%) for females and 3.5% (95% CI 1.9–5.2%) for males [37]. Other parts of the world, young females are also found with much higher estimated CT prevalence, e.g. in Peru, the CT prevalence in the 18–27 year-old sexually experienced women is estimated as high as 7.8% (95%CI 7.1–8.5%) [38].

At the same time, TeSSHS finds a higher prevalence of Chlamydia trachomatis with estimated prevalence of 4.1% (95% CI 1.8–9.0%) in the sexually active 40–49 year-old. All of them reported only one sex partner in the previous 12 months. These older women were mostly married and whose partners reportedly travelled out of Hong Kong within the past 12 months. TeSSHS also identifies partners travelling out of Hong Kong as a risk factor among the sexually experienced females, which is similar to that of the China CT study [16]. According to the data from SHC, the pattern of clinically CT cases shows two similar peaks for males, one at age 20–29 years and the other at age ≥40 years, whereas for females only the 20–29 years old peak is observed [39]. The mismatch of the SHC data and TeSSHS could be that the older females are normally reluctant to come forth for Chlamydia trachomatis testing as they may not be aware of their risks [4].

Naturally CT prevalence is much lower in the general populations than those in key populations. According to a study with cross-border Hong Kong-based Chinese male truck drivers in Hong Kong, CT prevalence is as high as 8.5% [40]. Another study among female sex workers in Hong Kong finds CT prevalence at 4.6% using CT DNA testing [41], which is higher than the estimation in the sexually experience but lower than that of the sexually active young women in our study. Nevertheless, a more recent CT prevalence estimation among the female sex workers in Hong Kong estimates the genital CT prevalence at 10.6% [28]. Over the Hong Kong-China border in Shenzhen, the combined prevalence of genital CT is reported to be as high as 17.7% in a sample of 2534 patients from 34 hospitals [42].

Other developed countries such as in the USA and UK have introduced opportunistic CT screening programs where young females are recommended to have CT tested regularly [8, 19]. A systematic review concludes that when prevalence of CT is between 3.1–10.0%, it can be cost-effective [43]. Based on the findings of TeSSHS, Chlamydia trachomatis screening for sexually active youth between 18–26 years of age and older women (40–49 years) could be cost-effective. Other risks factors such as living status and travel history could help identify people of higher risks. Certain key populations have expressed concern and the need of CT screening, for example, in the CT prevalence study among the cross-border truck drivers, a third of those who phoned for the results expressed this need [40]. The female CT positive participants indicate testing preference of public facilities, which could be related to costing issue, or their trust in public health system in Hong Kong. The public facilities could be ideal sites for health promotion, education and screening for chlamydia.

Supporting information

Acknowledgments

The authors would like to thank all the interviewers and the participants who have completed this study.