Epidemiology of hepatitis B infection in Hong Kong

Hepatitis B (HBV) infection is an important cause of acute viral hepatitis as well as chronic liver diseases in Hong Kong. Studies published in early 1990s have shown that some 85% to 90% of symptomatic hepatocellular carcinoma in Hong Kong had evidence of previous hepatitis B infection.[1]^,[2] Tracking the epidemiology of HBV can shed light on the magnitude of HBV-related diseases burden in the future to come. This has been achievable through the regular collection of statistics by health care or other institutions, and information input from designated studies. The Scientific Working Group on Viral Hepatitis Prevention of the Department of Health has been collating and publishing annual viral hepatitis surveillance report since 1996. This paper was abstracted from its 2002 Update Report, which was published in December 2003. (http://www.info.gov.hk/hepatitis/download/papers/hepsurv02.pdf)

Acute hepatitis B from the disease notification system

Hepatitis B comes after hepatitis A as the second commonest viral hepatitis notified. (Box 1). There were 100-200 cases reported annually in the last decade. This observed phenomenon is possibly explained by the all-along high endemicity of past HBV infection and the introduction of universal neonatal hepatitis B vaccination in 1988.

Hepatitis B markers in young adults

HBV is monitored through the testing of serological markers resulting from the infection.  The commonest markers monitored are:

1. HBsAg - present in chronic carriers and also during active acute infection;
2. Anti-HBs - indicative of immunity to the virus;
3. Anti-HBc - an indicator of natural infection. 

Data on hepatitis B markers in young adults have come mainly from the following ongoing programmes:

1. Blood donor screening at the Hong Kong Red Cross Blood Transfusion Service, 
2. Antenatal testing at the Department of Health's Maternal and Child Health Centres,
3. Pre-marital package service of the Family Planning Association of Hong Kong, and
4. Hepatitis awareness project of the City University's Health Centre and
5. Hepatitis awareness project of the Baptist University's Health Centre.

New blood donors

The majority of blood donors in Hong Kong are between the age of 16 and 30.  Data from the Hong Kong Red Cross Blood Transfusion Service (HKRCBTS) revealed a declining trend of HBsAg prevalence in this group of young adults, from 7.97% in 1990 to 3.63% in 2002 in new donors. (Box 2) However, this population may be a biased one because some known carriers may not go for blood donation, which could account for the low rate in the new donors.  The prevalence in repeat donors was 0.14%.

As shown in Box 3, there was no specific age pattern for HBsAg positivity among new blood donors in 2002.  However, the prevalence was consistently higher in male for all age groups, with an overall rate of 4.2% and 3.0% in male and female donors respectively.  The odds ratio of HBsAg positivity in male was 1.46 (95% CI, 1.31-1.62).

University students and staff

The prevalence figures obtained from the ongoing hepatitis awareness project for students and staff of the City University of Hong Kong from 1994 to 2002 (Box 4) were similar to those of the new blood donors. The highish 6% of HBsAg prevalence in those aged below 21 in 2002 may be related to the small number tested.

Since 2001, data from the Baptist University Health Awareness Programme (Box 5) was included.  In 2002, the HBsAg prevalence rate was 1.5% in persons below 21 years old and 2.3% in those aged 21-30.

Amongst 1328 students (aged 18-25) of the Chinese University of Hong Kong tested in 2001/2002, 3.6% were positive for HBsAg, with a corresponding rate of 3.2% in males (n=505) and 4.4% in females (n=823).  The HBsAg prevalence was 2.9% in 3457 secondary school students tested in 2001/2002.[3]

Clients of the Pre-marital package service of the Family Planning Association

The falling trend of HBsAg in young adults was also evident in data from the Pre-marital Package Service of the Family Planning Association. (Box 6)  The prevalence rates were comparable though somewhat higher than the HBsAg positive rates reported in new blood donors and young university students and staff.  However, the rates were in general lower than those of  antenatal mothers (see below).  In 2002, for example, the HBsAg prevalence of young adults attending the Family Planning Association was 6.9%, compared to that of 9.0% in the expectant mothers attending antenatal clinics.

Antenatal mothers

The HBsAg prevalence in antenatal mothers has also been falling over the years.  The observation carries significant implication as it could be used to predict the future trend of perinatal infection.  Results from antenatal screening demonstrated a steady decline from over 10% in the early 1990s to 9.0% in 2003 (Box 7).

Those between the age of 15 and 19 had a lower prevalence of 5.0%, compared to that of 8.1% above the age of 34 in 2002 (Box 8).  The results of clients younger than 15 years of age should however be interpreted with care because:

1. of the small number involved, and  
2. the dataset has actually included abortion cases and non-pregnant clients from sources other than the Maternal and Child Health Centres (MCHC) who attended for screening of sexually transmitted disease (STD)

Despite the young age of the antenatal population, the HBsAg rate was generally higher than that in new blood donors, young university students/staff and clients of pre-marital package.  One of the confounding factors may be the place of birth of the individual.  A study on 2480 pregnant women attending the MCHC in 1996 showed a difference in HBsAg positive rate between locally and non-locally born antenatal mothers[4].  Those born in Hong Kong had a HBsAg prevalence of 8.4%, versus that of 13.1% in those born in Mainland China.

Age and hepatitis B markers

There is a positive correlation between age and the prevalence of hepatitis B markers from natural infection in a population.  Generally speaking, the older a person is, the higher the chance of having been exposed to hepatitis B virus and thus harbouring markers of the infection.  This association was illustrated by the HBsAg figures for antenatal mothers in the early 1990s (Box 8 & 9).  However, this relationship disappeared after 1998, the reason for which is unclear.

Box 9. HBsAg prevalence among antenatal mothers of different age groups in 1990, 1995, 2000-2002 (Data source: Family Health Service & Virus Unit, DH)

Another more compelling piece of evidence that confirmed this finding was derived from the hepatitis B pre-vaccination screening done for the police force between 1996 and 2002 (Box 10 & 11).  The results demonstrated a positive correlation between age and HBV markers. Furthermore, the HBsAg positivity rate correlates with age in males.  In addition, similar to the new blood donors, men had a higher prevalence of HBsAg than women for all age groups (Box 11), or were tested positive for any HBV markers in a higher proportion of the subjects.  However, it must be cautioned against extrapolating these figures to the general population because it was a highly selected sample.

Box 11.  HBsAg prevalence among male and female police officers of different age groups from 1996 to 2002 (Data source: DH)

*The zero prevalence in the 51-60 years old female group could be result of the small number involved.

In the screening of laboratory samples left over from routine virological investigations in 2001, it was found that HBsAg was absent in those below 10 years old. However, anti-HBc rose markedly from 1.3% in persons of 1-4 years old to 7% in those of 5-9 years old (Box 12).

Hepatitis B serology in occupationally exposed professionals

Health care workers are at risk of HBV infection because of potential occupational exposure to blood and body fluids.  In 1983, a study in Hong Kong reported a higher rate of HBsAg in those who had been in service for over 10 years (10.8%) versus those at entry (7.5%)[5].   The rates for anti-HBs were 43.1% and 20.3% respectively.

The HBsAg prevalence was lower in subsequent studies.  In 1992 and 1993, HBsAg and anti-HBs were positive in 4.4% and 38.2% respectively of 5825 health care workers screened[6].  The corresponding figures were 7% and 36.3% in data collected in a vaccination campaign of the Department of Health in 1995.  Again, in all instances, the HBsAg prevalence varied positively with age.

As shown in Box 13, the prevalence of HBsAg in new recruits of public service health care workers in 2002 was 5.0%.  Again, the positivity rate was lower than those of earlier studies in the 1980s.  However, since Sep 2000, HBV screening procedure of new recruits has been changed from an opt-in to opt-out approach. Thus, this change could have affected the HBV results in 2001 and 2002, as compared with earlier years.

Risk behaviours and Hepatitis B markers

Unprotected sex and needle sharing (in injecting drug users) are known routes of HBV transmission. Three programmes offered data on hepatitis B infection in people who were more likely to have engaged in high risk behaviours predisposing to HBV transmission - drug users, commercial sex workers and HIV-infected patients.

Drug users

Tests for hepatitis B markers were offered to drug users who had registered with methadone clinics or other drug rehabilitation services.  Box 14 shows the prevalence of various hepatitis B markers among drug users in the last 13 years.   HBsAg positivity rate has gradually fallen from over 13% in 1990 to a nadir of 6.6% in 1997.  However, the rate then increased again to about 11% in 1999 and remained similar at around 12-13% in the last 2 years.  In the past, around 90% of drug users were positive for at least one of the three markers (HBsAg, anti-HBs and anti-HBc); this has dropped to 53.4% in 1997 and then slowly rose over the last few years, to 72.3% in 2002. It must be cautioned that the number of drug users surveyed from 1995 to 1998 was small compared with other years, and the data were collected from multiple sources. In general, HBV markers were still detected in a large proportion of drug users. 

Female commercial sex workers

From 1995 to 1998, the government Social Hygiene Service which provides free treatment for sexually transmitted diseases conducted a study to examine the prevalence of hepatitis B markers in female commercial sex workers in Hong Kong.  The complete study had involved a total of 1020 female commercial sex workers recruited at one Social Hygiene Clinic on Kowloon side.  The prevalence of the serological markers was: 69 (6.8%) positive for HBsAg; 551 (54.0%) positive for anti-HBs; and 400 (39.2%) negative for either. An analysis on 100 commercial sex workers was published[7].

HIV-infected patients

Testing for HBV markers has been offered to clients attending the HIV clinic of the Department of Health.  As HIV shares the same routes of transmission with HBV, it is not surprising to find a high HBsAg positivity rate in newly seen HIV-infected patients - 9.3% (2000), 10.9% (2001) and 10.6% (2002). (Box 15) Again, the rate was substantially higher in male than female.

Current situation in general population 

The Viral Hepatitis Preventive Service of the Department of Health, the Department of Microbiology of the University of Hong Kong and the Department of Paediatrics of the Pamela Youde Nethersole Eastern Hospital conducted a territory-wide Community Research Project on Viral Hepatitis (CRPVH) 2001 to study the epidemiology of viral hepatitis in Hong Kong.  Through a standardised telephone sampling survey, Chinese-speaking household members aged 18 or above were interviewed in 2001.  Of the 5017 successful telephone respondents, 1610 agreed to attend and eventually 936 (18.7%) turned up for blood screening.

The HBsAg prevalence is shown at Box 16, with age and gender breakdown.  The overall prevalence was 8.8% (95% CI, 7.1% to 10.7%), with again a higher rate of 10.4% for male as compared with 7.7% for female.  There was no definite age pattern observed in this study.  Unexpectedly, the rate was only 5.8% in those aged over 50.  Two (2.5%) of the 81 HBsAg positive persons were anti-HDV positive, representing 0.2% of the total cases.  The results have to be interpreted with caution as the blood-screened subjects were significantly over-represented by people aged 30-59, those who had received formal schooling and people without paid work. 

In another study conducted by the Chinese University of Hong Kong on subjects attending a Regional Council Health Festival for general public, the HBsAg positivity rate was 8.3% for 1929 people of 18-60 years old; the respective prevalence in male and females were 10.8% (n=539) and 7.3% (n=1390).[3]

Conclusions

An obvious pattern observed was the decline of hepatitis B markers in most of the community groups studied over the past years.  The decline was observed in new blood donors, university students/staff, and police officers. The drop was less obvious in antenatal mothers - HBsAg rate remained high at about 9% from 1999 to 2002. As regards populations with high-risk behaviours, the HBsAg positive rate in drug users has generally fallen over the years until 1997, followed by a slowly rising trend again (12.7% in 2002). The prevalence rate in this population is still substantially higher than the general population. Hence, whereas it is still customary to quote an HBsAg carriage rate of 10% in Hong Kong, evidence has emerged to support that it could be much lower. Age is generally an important factor affecting HBsAg prevalence, with a higher proportion of the older population having markers of past infection or becoming chronically infected. 

It can be inferred that perinatal infection has been the commonest cause of HBV transmission in Hong Kong, based on the observation that HBsAg was high in young adults in the general population. With the universal neonatal hepatitis B vaccination programme in place since 1988, infection and carriage in childhood would most likely continue to decline.  This was supported by the absence of HBsAg positivity in samples obtained from children below 9 years old in 2001. There is a possibility that sexual contact may become the next common mode of HBV transmission in the future. 

References

1. W Shiu, Dewar N, Leung N, et al. Hepatocellular carcinoma in Hong Kong: clinical study on 340 Cases. Oncology 1990;47:241-245. 

2. C.L. Lai, JYN Lau, PC Wu, et al. Subclinical hepatocellular carcinoma in Hong Kong Chinese. Oncology 1992;49:347-353. 

3. Leung NW, Tam JS, Kan P, Chan L, Chu KW. Decline in viral hepatitis B infection in Hong Kong in the past 20 years:1983-2002. [Abstract] AASLD 2002, 53^rd Annual Meeting and Postgraduate Course, Boston, USA, 1-5 Nov 2002.

4. Kwan LC, Ho YY, Lee SS. The declining HBsAg carriage rate in pregnant women in Hong Kong. Epidemiol Infect 1997;119:281-283.

5. Yeoh EK, Lo HY, Chang WK, Lee SH. Hepatitis B vaccination in health care personnel in a region of high prevalence. Hepatology 1983;3:1079 [abstract]

6. Lim WL, Wong DA, Cheng KC. Immune response to hepatitis B vaccine in health care workers in Hong Kong. HK Med J 1996;2:138-140.

7. Yu CW, Chong LY, Lo KK, Ng PS. The epidemiology of hepatitis B infection in commercial sex workers in Hong Kong. Southern China J Dermato-venereology 1998;5:47-49. [in Chinese]