The discrepancy in incidence of Hib disease between local Hong Kong Chinese and the Vietnamese refugees could be the result of environmental, genetic or other unknown factors. The high incidence rate of Hib disease in the Vietnamese refugees suggests that the low incidence rate in the local Chinese cannot be attributed to inadequate laboratory facilities for isolation and serotyping, because both groups of children were served by the same hospitals. However, the medical care of the Vietnamese refugees was tightly controlled, whereas the local Chinese had ample opportunities to obtain antibiotics over the counter or through private practitioners before presenting to hospitals. This indiscriminate use of antibiotics among the local Chinese might interfere with culture of the bacterial agent responsible for infection.14 In contrast the Vietnamese refugees might have little or no pretreatment with antibiotics before admission to hospital. This possibility is supported by the observation that 48% of the 129 children with postneonatal bacterial meningitis admitted to a teaching hospital in Hong Kong between 1981 and 1988 had no growth in the CSF.13 The three most common organisms obtained in culture were Salmonella species (n = 16), H. influenzae (n = 14) and Streptococcus pneumoniae (n = 14).13 Similarly in Thailand up to 60% of cases of bacterial meningitis in young children were of unknown etiology.15 Antibiotic abuse is still a common problem in Asia14, 15 and might partially explain the "low" incidence of Hib disease in this part of the world.
Overcrowding in the Vietnamese refugee camps might be another factor for their high incidence of Hib diseases. Their living area per person was 1.71 m2, compared with 10.20 m2 for the local Chinese.16 These are extremely crowded urban conditions when compared with those in the developed world.
The observation that 14 of the 39 Chinese patients and only 1 of the 18 non-Chinese patients had preexisting medical problems (P = 0.022) suggests that extra risks must be present in Chinese for Hib disease to occur, e.g. prolonged hospital care for underlying medical problems. The coexisting medical problems also explain the high fatality rate of the Chinese patients (12.8%) compared with that of the non-Chinese patients (5%).
The apparently low incidence of Hib diseases among the local Chinese could be the result of unreported cases or culture-negative cases. Even ascribing all the culture-negative cases to Hib, the incidence would have been increased by only 4- to 5-fold, i.e. 10.8 to 13.5 per 100 000 children <5 years of age. This incidence rate is still lower than that reported in the West before the introduction of universal Hib vaccination, but quite comparable with that reported in a recent prospective study done in Hefei City, China.12 There may be genuine and significant differences in incidence of Hib disease between Chinese and Caucasians, but further studies are mandatory to resolve this issue.
Attempting to explain the apparently low incidence of Hib diseases in Hong Kong, 2 studies investigating the carriage rate of Hib were performed.16, 17 In a study of 621 healthy Chinese children and 300 healthy Vietnamese refugees ages 2 months to 5 years in Hong Kong, the nasopharyngeal carriage rate of Hib was found to be zero in Chinese and 1.3% (95% CI 0.04% to 2.63%) in Vietnamese refugees.16 The carriage rate of nontypable H. influenzae was 5.8% (95% CI 1.4% to 7.6%) in Chinese and 65.4% (95% CI 58.9% to 69.8%) in Vietnamese.16 The carriage rates of both type b and nontypable H. influenzae in Vietnamese refugee children were similar to those reported from other parts of the world,18-20 whereas the rates in local Chinese children were extremely low, with Hib being absent from 802 swabs from 621 children.16 The possibility of sampling or laboratory errors was considered unlikely to explain the low rate among the Chinese children because a much higher carriage rate was found in the Vietnamese refugees and the same staff and techniques were used for both groups of children. Ethnic group was the only significant predictor for the carriage of Hib (P = 0.0013). For the carriage of non-type B H. influenzae, three factors were significantly associated with higher carriage rates: Vietnamese race (odds ratio, 27); age >3 months vs. <3 months (odds ratio, 3.4); and having >2 siblings (odds ratio, 3.7).
It has been argued that nasopharyngeal swabs might underestimate the carriage rate of Hib when compared with throat swabs. However, in a larger study of Hib and non-type b H. influenzae carriage rates among 1812 healthy Chinese children ages 6 months to 5 years investigated by throat swabs, again no Hib was isolated but 141 children (7.8%) were found to be carriers of non-type b H. influenzae.17 The low carriage rate could be the result of sampling error because this was only a cross-sectional study rather than a longitudinal study. Nevertheless the 2 carriage rate studies corroborated each other,16-17 and the findings reflected the discrepancy in the incidences of invasive Hib disease between the local Hong Kong Chinese children and Vietnamese refugee children.13
In a study of age-related natural anti-Hib antibody among 596 healthy Chinese children and adults in Hong Kong, the antibody profile was found to be similar to that in Finland; i.e. only 25% had the protective level of >0.15 μg/ml by 1 year but 90% had reached >0.15 μg/ml by 6 years of age.17 This finding would suggest exposure to the Hib capsular polysac-charide in Hong Kong Chinese similar to that in the Finns. However, the extremely low carriage rate of Hib among Hong Kong Chinese would suggest otherwise. Indeed there was some evidence that these "natural" antibodies against Hib in the Hong Kong Chinese were cross-reacting antibodies against antigens on other encapsulated bacteria.21
The documentation of disease burden caused by Hib in Hong Kong is still far from satisfactory. The limited number of published studies thus far could underestimate the magnitude of Hib diseases because of widespread antibiotic abuse, study design or some other unknown factors. It is essential to refine the documentation of the disease burden in various Asian countries, perhaps using a common protocol prospectively. Thus far the evidence suggests that Hib disease occurs at a lower absolute level in Hong Kong than in the West. Nevertheless the relative importance of Hib in causing childhood bacterial meningitis has been well-documented in Hong Kong and several other Asian countries. This single observation should be powerful enough to urge public health authorities to put universal Hib vaccination on their agenda for urgent discussion.
We thank all of the members of the Hong Kong Hib Study Group: Drs. A. K. H. Chan (Caritas Medical Centre); G. Chan and W. H. Lee (Queen Elizabeth Hospital); C. B. Chow and C. W. Leung (Princess Margaret Hospital); A. Ho, S. J. Oppenheimer and R. Sung (Prince of Wales Hospital); P. Ip (United Christian Hospital); E. Kwan, Y. L. Lau, L. Low and R. Yung (Queen Mary Hospital); K. W. Ng (Department of Statistics, University of Hong Kong); and R. Yuen (Kwong Wah Hospital).
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FIRST INTERNATIONAL CONFERENCE ON HAEMOPHILUS INFLUENZAE TYPE b INFECTION IN ASIA
The Editors thank the Association pur l'Aide à la Médicine Préventive, the Foundation Mérieux, and the World Health Organization for supporting publication of these proceedsings, and Jennifer Wells for her editorial assistance.