CW Leung

Hong Kong has witnessed tremendous decline in the incidence and mortality of major childhood communicable diseases over the past three decades. The pivotal role of immunization in bringing about this change in infectious disease epidemiology cannot be over emphasized.

The childhood immunization programme of Hong Kong has come of age since its introduction in the 1950's. The Advisory Committee on Immunization, chaired by the Deputy Director of Health, formulates and periodically reviews the immunization strategies in Hong Kong. Today, immunization for infants and children are targeted against nine vaccine-preventable diseases - tuberculosis, diphtheria, tetanus, pertussis, poliomyelitis, hepatitis B, measles, mumps and rubella. The first seven are included in the EPI promulgated by the WHO for this region.

The 1998 Recommended Immunization Programme (Tables 1 and 2) comprised of the Preschool Immunization Programme (newborn to 5 years of age) and the School Immunization Programme (Primary 1 to Primary 6, or about 6 to 12 years of age). The Family Health Service and the Regional Health Offices of the Department of Health are responsible for the provision of immunization services to children. Private medical practitioners also contribute significantly to the successful delivery of vaccines to children. Immunization is provided free of charge to newborns in public hospitals, at Maternal and Child Health Centres for preschool children under 5 years of age, and in primary schools by mobile immunization teams of the Regional Health Offices. BCG booster immunization of school children is however provided by the Tuberculosis and Chest Service of the Department of Health. Primary school children will receive BCG booster if they were found to be tuberculin negative by Mantoux testing. BCG and Type 1 OPV are supplied by the government free to the private sector.

Table 2 Recommended Immunization Programme 1998

The achievement of the immunization programme in Hong Kong is reflected by the high coverage rates of various vaccines (Tables 3 and 4). About 99% of newborns have received BCG, Type 1 OPV and the first dose of hepatitis B vaccine (HBV). Over 80% of preschool children have completed the prima series at Maternal and Child Health Centres. An additional 10% of preschool children have been immunized by private medical practitioners. The immunization coverage rates for Primary 1 and 6 school children have been maintained consistently above 99%, "Mop-up" exercise for those school children with incomplete primary immunization also forms an integral part of immunization campaign schedule for schools which currently include two visits each year.

In anticipation of a projected measles epidemic in 1998 basing on epidemiological observations, the Advisory Committee on Immunization has recommended a one-off Special Measles Vaccination Campaign (using MMR vaccine) targeting individuals aged 1 to 19 years, which was conducted from 28 July to 13 December 1997. The coverage rate for individuals who neither had had natural measles nor two doses of measles vaccination before was 76.85%, covering 1.1 million recipients. By boosting individual immunity and increasing herd immunity to an estimated level of 96.8% against measles for ages 1-19, Hong Kong has come one step closer to what has already been achieved in Finland, where measles is declared to be eradicated.

Despite a high coverage rate of over 99% for the first dose of HBV for newborns born in Hong Kong, the figure for the third dose was only around 85%. In addition, recent immigrant children may not have received any hepatitis B vaccination at all. A Supplementa Hepatitis B Vaccination Programme was thus launched starting the 1998-99 school year, targeting Primary 6 students who had not received or not yet completed the three-dose regimen. This is to supplement the universal neonatal hepatitis B vaccination programme started in 1988, and the one-off campaign in 1993 to immunize children born between 1986 and 1988. The exercise ensures that all children will have received adequate protection against hepatitis B before they grow up to become sexually active, when hepatitis B is commonly transmitted through sexual contact. All eligible students would be immunized and followed up to complete the whole course of vaccination.

BCG revaccination continues to be practised in Hong Kong. The WHO however does not advocate the practice of using tuberculin skin test to formulate decisions on BCG vaccination, and revaccination was also not recommended. In fact, neighbouring countries like Taiwan, Korea and some cities of China like Shanghai have already dropped their BCG revaccination programmes. A review of the cost-effectiveness of existing BCG vaccination programme has become a pressing need.

Currently, a typical child born in Hong Kong is recommended to receive at least 12 doses of parenterally administered and 6 doses of orally administered vaccines before he / she reaches his / her twelfth birthday. The total number of vaccine doses is expected to increase further with better understanding of booster requirement and availability of newer vaccines with proven tolerability, immunogenicity, durable protective efficacy, improved safety profile and temperature stability.

Vaccines that have recently become available include acellular pertussis vaccine, inactivated polio vaccine with enhanced potency (eIPV), Haemophilus influenzae type b (Hib) conjugate vaccine, hepatitis A vaccine, varicella vaccine, rotavirus vaccine, and pneumococcal conjugate vaccine. In time it is likely that they would be put on the agenda for consideration by the public health authorities.

Acellular pertussis vaccine is unlikely to replace its inexpensive whole cell counterpart which has proven efficacy in mass immunization programmes. Though less reactogenic, the cost of acellular pertussis vaccine (marketed as DTaP) is prohibitive in view of its marginal cost-benefit ratio. Changing an already well established cost-effective immunization practice (using DTwP) within a stretched health budget would seem unwise. Moreover, as DTwP vaccine was well tolerated and attained good coverage the uptake rate is unlikely to be increased by a switch-over to DTaP vaccine.

With the impending global eradication of poliomyelitis, the risk of vaccine-associated acute flaccid paralysis in recipients of OPV might in fact outweigh the risk of acquiring wild-type polio infection. The need to consider switching from OPV to IPV or eIPV in order to avoid putative vaccine-associated paralysis has become more pressing than ever.

Private practitioners in Hong Kong have not hesitated in introducing or recommending Hib, varicella and hepatitis A vaccines to their prospective clients. A retrospective survey conducted by the Hong Kong Hib Study Group has established that the annual incidence of invasive Hib diseases for children less than 5 years of a e was 2.7 per 100,000 (95 CI, 2.0 to 3.5). in addition, two prospective studies have virtually confirmed the lack of nasopharyngeal carriage of Hib amongst local children aged 5 years and under. Although the documentation of disease burden caused by Hib is still far from satisfactory, it is apparent that the risk of colonization or infection with Hib is dismal. Universal immunization of infants against Hib would not seem to be cost-beneficial and there is a lack of urgency at the moment. Currently, the International Vaccine Institute based in Seoul, Korea is coordinating a regional comprehensive surveillance for invasive Hib disease to better define the need of introducing Hib conjugate vaccine into national immunization programmes of East and Southeast Asia. Forthcoming data will help clarify whether the problem has always been underestimated in this part of the world.

The cost-benefit analysis of introducing universal varicella immunization for local children is not an easy undertaking. Without accurate notification and hospitalization figures for this generally benign childhood infection, the quantification of both the direct and indirect costs of varicella can be difficult. Preliminary assessment by the Advisory Committee on Immunization has concluded that universal varicella immunization is deemed only marginally cost-beneficial at the moment. Of course, the issue could be revisited in light of the changing epidemiology of varicella and its complications, if there happens to be any.

For hepatitis A vaccine, mass immunization of children (or adolescents) is worth considering. In Hong Kong, hepatitis A is changing from an infection of intermediate endemicity to one of low endemicity, and the percentage of non-immune young adults has steadily increased over the past two decades. Certainly, the vaccine is far from being just a travel vaccine for providing individual protection. The question is how best to use it. The Advisory Committee on Immunization has resolved that the cost-effectiveness of the vaccine in mass immunization has yet to be convincingly demonstrated. Personal, environmental and food hygiene remain the cornerstones of prevention.

To prioritize immunization strategies against diseases which have recently become vaccine-preventable would require comprehensive surveillance of local disease burden and mathematical modeling of the cost-effectiveness of corresponding vaccination programmes. The road to final adoption of any immunization strategy (e.g. rotavirus vaccine for infants or pneumococcal conjugate vaccine to combat emerging penicillin resistance), or the decision not to adopt any, is never an easy one. It is hoped that with vaccines incorporating multiple antigens at a minimal extra cost that our future generation would benefit from the promise of a newer generation of vaccines. In this regard, vaccines like measles-mumps-rubella-varicella (MMRV), DTP-HBV, DTP-eIPV, DTP-Hib conjugate would seem to be a good start. Until then, we will stick to time-honoured traditional childhood vaccines, at least at the beginning of the new millenium.