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.
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
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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.