B06 Public health Surveillance

Introduction

Public health surveillance is the continuous, systematic collection, analysis and interpretation of health-related data needed for the planning, implementation, and evaluation of public health practice. It can serve as an early warning system for impending public health emergencies, document the impact of an intervention, track progress towards specified goals, monitor and clarify the epidemiology of health problems, allow priority setting and inform public health policy and strategies.[1] HIV surveillance is a regular and continuous process, involving four major steps – data collection; data collation and analysis; information dissemination; and HIV prevention programme planning, implementation and evaluation, as illustrated in Box 6.1.

With HIV being one significant global public health challenge, surveillance of HIV/AIDS plays an important role in the control and monitoring of the epidemic. This chapter covers the layout of the main components of the HIV surveillance process in Hong Kong.

Surveillance methodology

The World Health Organization (WHO) has recommended Second Generation Surveillance strategy for HIV/AIDS.[2] Second generation surveillance for HIV/AIDS is the regular, systematic collection, analysis and interpretation of information for use in tracking and describing changes in the HIV/AIDS epidemic over time. The strategy also involves gathering information on risk behaviour, using them to warn of or explain changes in levels of infection. As such, second generation surveillance includes, in addition to HIV surveillance and AIDS case reporting, sexually transmitted infections (STI) surveillance to monitor the spread of STI in populations at risk of HIV and behavioural surveillance to monitor trends in risk behaviour over time. These different components achieve greater or lesser significance depending of the surveillance needs of a country, determined by the level of the epidemic it is facing: low level, concentrated or generalised. [Box 6.1]

UNAIDS defines HIV epidemic as ‘concentrated’ when HIV prevalence is over 5% in any sub-population at higher risk of infection, which includes people who inject drugs (PWID), female sex workers (FSW), men who have sex with men (MSM)). In Hong Kong, the HIV prevalence among MSM was found to be 5.85% and 6.54%, according to the latest HIV and AIDS Response Indicator Survey (HARiS) conducted in 2014 and HIV Prevalence and Risk behavioural Survey of Men who have sex with men in Hong Kong (PRiSM) in 2017 respectively.[2][3] [Chapter A2] For male-to-female transgender, the HIV prevalence was found to be 5.11% in PRiSM 2017. In this regard, the HIV epidemic in Hong Kong is considered as concentrated. As recommended by WHO, surveillance systems are meant not only for monitoring infection in those at risk, but also assessing the behavioural links between members of high risk population groups and the general population. The systems should also monitor the general population, especially young people, for high-risk sexual behaviour that might lead to rapid spread of the virus if it were introduced and trends in STI.[4]

Components of the HIV surveillance system in Hong Kong

With reference to the WHO’s recommendations, the HIV surveillance system of Hong Kong comprises the following components : [Box 6.2] –

1. Size estimation of key populations at higher risk
2. Biobehavioural surveys of key populations at higher risk
3. HIV and advanced HIV infection case reporting
4. STI case reporting
5. Facility- or community-based HIV and STI sentinel surveillance for key populations at higher risk

Size estimation of key populations at higher risk

A crucial step of HIV/AIDS surveillance is to identify key populations at increased HIV risk from high-risk behaviours, which are:

1. men who have sex with men (MSM)
2. people who inject drugs (PWID)
3. male-to-female transgender (TG)
4. female sex workers (FSW) and
5. male clients of female sex workers

In Hong Kong, MSM has predominated new HIV infections since 2009. In 2017, homosexual and bisexual contacts (i.e. MSM) as route of transmission contributed to 67% of all HIV reported cases and 77% in all male cases. The number of HIV reports from MSM has been persistently higher than that from heterosexual men since 2005 and the trend continued to widen in recent years. In contrast, heterosexual male cases showed a decreasing proportion in past few years (10% of all male cases in 2017). For PWID, HIV can be transmitted through sharing of needles among them. There have been many examples of HIV outbreaks in various countries among PWID populations.[5] Therefore, despite the fact that no more than 15 cases arising from injecting drug use were reported annually in the past few years in Hong Kong, with the high needle – sharing rate of 17.6% (HARiS 2017), there is a need to remain vigilant with this population. For TG, the HIV prevalence of TG was found to be 5.1% (PRiSM 2017) but as high as 18.6% for TG sex workers (HARiS 2014) (small sample size of 43). Their consistent condom use rate, defined as always using a condom for anal/vaginal sex in the preceding 6 months, with men was low at 30.8% for receptive sex and 23.3% for insertive sex (PRiSM 2017). The clients of TG and female sex workers can be regarded as a ‘bridging population’, who have the potential to transmit HIV among their clients and even the general population.

UNAIDS has recommended various methods for the estimation of the size of key populations.[6] In Hong Kong, the AIDS Epidemic Model (AEM) has been applied for epidemiological modelling.[7] This modeling work was parametrised by case reports, cross-sectional surveys (for example, PRiSM, HARiS, number of individuals on antiretroviral therapy and laboratory identification of recent infections) and the results are consistent with analyses from other sources of data collected in Hong Kong over the course of the epidemic.

Biobehavioral surveys

In theory, HIV transmission is determined by the chance of being infected through a specified behaviour with a partner who is HIV positive (e.g. unprotected anal sex), chance of partner being HIV infected (e.g. prevalence of HIV among MSM community) and the frequency of the act (e.g. frequency of unprotected anal intercourse). In order to appreciate the dynamics of HIV transmission, an understanding of the changing pattern of the implicating risky behaviours is crucial.[8] In this connection, the monitoring of behavioural risks of the high risk populations, together with the sub-population prevalence, could provide early warning signals of HIV spread for informing timely response.

For continuous surveillance purpose, regular behavioural surveys are conducted in selected target populations in different settings, including PWID recruited in drug rehabilitation centres and methadone clinics, newly admitted inmates of prisons, clients of Social Hygiene Service of the Department of Health (DH), AIDS Counselling and Testing Service clients and MSM undergoing HIV testing in community organisations. Box 6.3 lists out all the behavioural studies concerning HIV conducted in Hong Kong since 1972. These surveys were supplemented by periodic behavioural surveys administered on MSM and FSW: the PRiSM surveys conducted in 2006, 2008, 2011 and 2017; HARiS every year since 2013; the Community Based Risk Behavioural and Seroprevalence Survey for FSW in Hong Kong (CRiSP) in 2006 and 2009.

The HIV-related behavioural markers used in different populations may vary but are largely similar in broad areas, which include basic demographic data (age, sex, ethnicity); behavioural risk factors (consistency of condom use, needle sharing rate, HIV testing pattern, number of sex partners etc); and access to HIV/STI prevention information. There are always problems in ensuring consistency of survey mechanisms, in order to make comparison between surveys in the same sub-populations and subsequent construction of a concrete picture possible. To improve the representativeness of the HIV/AIDS behavioural surveys, standardisation of behavioural indicators remains a challenge.

HIV/AIDS case reporting system

The HIV/AIDS case reporting system in Hong Kong has been in place since the 1980s. All doctors are encouraged to report patients with HIV/AIDS and to update status of previously reported cases where appropriate by completing the standardised reporting form (DH2293, available at DH Virtual AIDS Office website: http://www.info.gov.hk/aids/english/surveillance/form.pdf). This is an anonymous and voluntary reporting mechanism, with the collected data treated in strict confidence.

Apart from basic demographic information, the reporting system captures specific data including speculated route of transmission, suspected location of infection, AIDS defining illness [Box 6.4], CD4 level at diagnosis, and date of last negative HIV test (if known). Under the reporting mechanism, a positive HIV case is defined as one that has been tested positive by screening and confirmation. The Public Health Laboratory Centre of Department of Health is providing free confirmation testing service for samples tested positive by screening test, regardless of source (private or public). This is a mechanism to ensure the robustness of the reporting system. To achieve effective surveillance, collaborative effort of stakeholders is essential, including doctors, laboratories, government and private clinics, non-government organisations, Hong Kong Red Cross Blood Transfusion Service, drug rehabilitation institutions, prisons, members of the high-risk populations and also people living with HIV. An integration of data from multiple sources is necessary for analysis to be synthesised.

The case reporting system [Algorithm 6] is useful to track the number of new infections, routes of transmission, gender/age/ethnicity distribution, testing pattern, follow-up at HIV clinics, disease stage / timeliness of diagnosis as inferred from CD4 level and progression to AIDS among reported cases.

Sexually transmitted infections (STI) caseload statistics

As one of the sexually acquired infections, HIV surveillance could benefit by incorporating data from STI surveillance. It is known that the occurrence of other STIs increases the likelihood of HIV transmission through sexual contact.[9] In addition, presence of an STI itself is also an indicator of having high risk sexual behaviours. Prevention and effective treatment of other STIs have positive effects on HIV prevention efforts. Hence, STI surveillance can be considered a component of a comprehensive HIV surveillance programme as it can offer important insights into the trend of HIV epidemic.[10] In fact, linkages between HIV and STI prevention efforts are needed in order to control both epidemics.

Like HIV, STI is not a statutorily notifiable disease in Hong Kong. As a major provider of STI treatment service, Social Hygiene Service (SHS) of the Department of Health maintains the clinical caseload statistics of patients, which can be used to track the patterns of STI in the community. On a regular basis, periodic surveys are conducted to complement with behavioural data. Results of STI surveillance system are disseminated through the quarterly STD/AIDS updates (https://www.aids.gov.hk/english/surveillance/stdaidsupdate.htm)

Facility-based HIV and STI sentinel surveillance for key populations at higher risk

Facility-based HIV and STI sentinel surveillance for key populations at higher risk is only meaningful if this can be carried out in intervention sites serving more than 500 beneficiaries (among key populations at higher risk) annually.[4] This type of surveillance is only possible when large numbers of beneficiaries regularly access services at a community site. However, currently in Hong Kong, no such sites could be identified.

Other surveillance activities

As HIV continues to spread around the world, it became apparent that the epidemic does not follow the same course in all societies. As such, the scope of surveillance activities needs to be broadened to cover a wider range of areas in order to provide a more complete picture of the local situation. In Hong Kong these include HIV seroprevalence surveys and HIV-1 genotyping studies.

HIV seroprevalence surveys

HIV seroprevalence involves the measurement of prevalence of HIV infection in a defined population, often by HIV antibody testing. It can be conducted in communities and settings purposely or as a secondary output [Box 6.4]. Methods of acquiring seroprevalence include voluntary HIV testing and unlinked anonymous screening (UAS).[11] [12] Blood is the standard specimen, but other body specimens such as urine, has also been collected for surveillance.

The populations assessed through seroprevalence surveys can be broadly categorised by HIV risk: (a) community without obvious risk factors, e.g. blood donors, pregnant women receiving antenatal care, (b) community with predisposing risk factors, e.g. MSM, FSW, patients attending STI clinics, PWID, and (c) communities with unclassified risk, e.g. patients with tuberculosis (TB), prisoners.

UAS is one of the means for seroprevalence studies, which refers to “the testing of specimens for markers of infection after elimination (unlinking) of all personal identifying information from each specimen.” UAS minimises selection and participation bias. Nevertheless, there are shortcomings arising from the inability to link patients tested positive to appropriate care and treatment and there may be legal and ethical implications. However, these can be addressed by sticking to two key principles of UAS: (a) the result of a specimen cannot be traced back to its source, and (b) voluntary HIV testing for client’s own diagnosis being in place.[HK Guidelines 6A] In Hong Kong, UAS is conducted among newly admitted prisoners and inpatients of drug rehabilitation centre, where urine specimens are routinely collected for other purposes, for example, opiate screening. The confidentiality is protected by anonymous and unlinked testing.

HIV-1 genotyping studies

HIV is one of the fastest evolving microorganisms known. Several genetically distinct subtypes are present and new circulating recombinant forms (CRF) are continuously emerging.[13] Tracking of HIV-1 subtype has been pursued by researchers as early as 1996. It provides a systematic method for evaluating the introduction of new strains and movements of various strains across geographic regions, and facilitating prompt epidemiological investigation of unusual strain or transmission pattern. [Chapter A2]

To complement the surveillance system in Hong Kong, HIV-1 subtype determination was first explored in 2000. HIV-1 subtyping is performed using a standard laboratory method and followed by phylogenetic analysis to examine sequence variation and relationship. Genotyping findings are regularly fed into the HIV surveillance system. HIV molecular studies carry special meaning in Hong Kong, a concentrated epidemic area, as it enhances the sensitivity of the overall surveillance system by, for example, providing early signals of rapid local spread of HIV or any new/resistant strain circulating among MSM.

Reference

HK Guidelines