National influenza experience in Hong Kong, 1968

Bull. Org. mond. Sante 1969, 41, 349-351Bull. Wld Hlth Org. National Influenza Experience in Hong Kong, 1968 W. K. CHANGI Although its origin is uncertain, the 1968 influenza epidemic in Hong Kong may have spread from the mainland of China. It began in Hong Kong on 13 July and reached its maximum intensity in 2 weeks, lasting some 6 weeks in all. About 15 % of the population was affected, but the mortality rate was low and the clinical symptoms were mild. The causative strain was isolated on 17 July and, because of its antigenic deviation from 1967 A2 strains, was sent to the World Influenza Centre in London and the International Influenza Center for the Americas in Atlanta, Ga., It was then proved to be a distinct antigenic variant of A2 virus, and the World Health Organization warned of its possible world-wide spread on 16 August. The rapid spread of influenza virus variants within and from Hong Kong is facilitated by the overcrowding of the population and by the constant communication both with the Chinese mainland and with the rest of the world. The population density is such that an epidemic can occur even in the hot subtropical summer. Ever since our virus laboratory started functioning as a National Influenza Centre of the World Health Organization in 1963, we have been aware of the emergence of influenza virus mutants in this part of the world. Eleven years after the Asian influenza epidemic, a new virus variant was isolated in the summer of 1968 in Hong Kong. The origin of this variant is not known. There was no official informa- tion on an influenza epidemic from the health authorities of mainland China, but prior to the outbreak in Hong Kong, travellers reported an increased incidence of influenza-like infections in the neighbouring Chinese province. For various reasons, virus isolations were not carried out on arriving travellers to confirm these reports. Hong Kong is one of the few places which com- municates freely with the Chinese mainland. Cargo boats and trains daily bring in food supplies as well as passengers. Local residents are free to go to their native villages and return, provided they possess a re-entry permit. Hence, the place is vulnerable to the spread of influenza in the event of an epidemic in mainland China. On the other hand, Hong Kong, being a free port and a busy tourist centre, is also an effective place for virus exchange with other parts of the world by air and sea. 1 Senior Medical Officer; Government Virus Unit, Queen Mary Hospital, Medical and Health Department, Hong Kong. Overpopulation in Hong Kong facilitates rapid virus dissemination and renders an epidemic readily recognizable. Hong Kong has a population of 3.9 million and two-thirds of the population are concen- trated in the urban districts on the southern part of the Kowloon Peninsula and along the northern coast of the Hong Kong Island. The total area of the urban districts is about 5 mi2 (13 kM2) in which 1 million people live in the Government resettlement estates with a provision of about 30 ft2 (2.8 m2) for an adult. In urban areas, the population density is 500 persons per acre (123 500 persons per kM2) on av- erage. In these conditions of crowding, the seeding of a new virus variant is rapid and is almost imme- diately followed by an explosive outbreak. The mechanism of spread is undoubtedly by droplet from person to person and virus dissemination is most effective under all circumstances: in crowded public transports, in public places and places of work, in schools as well as at home. Most of the susceptible population is affected successively with- in a short period. The spread is not hindered by the hot weather of the subtropibal summer and the epidemic sweeps rapidly through the population. Influenza is not a notifiable disease in Hong Kong but voluntary notifications 2 on influenza-like dis- ' Data on influenza notifications reported in this paper were obtained from the Statistical Unit, Medical and Health Department, Hong Kong. 2373 - 349 - W. K. CHANG FIG. I SEASONAL DISTRIBUTION OF INFLUENZA-LIKE DISEASES IN HONG KONG, 195Q-67 AND 1968 eases are obtained from 9 of the government clinics. Whenever an increase of influenzal infections is reported, the laboratory carries out virological confirmation. In the period between 1962 and 1967, influenza outbreaks were recorded mostly in winter and early spring and less frequently in autumn, as is shown in the table. The influenza virus isolated on each occasion belonged to either type A2 or type B, which showed close antigenic relationship with the current virus strains. In contrast to the cold-season occurrence of influ- enza epidemics in Hong Kong, an epidemic broke out in mid-summer of 1968 (Fig. 1). It was first observed on 13 July, when there was a sudden in- crease of patients with influenza-like symptoms at the Government clinics. The epidemic soon reached MONTHLY NOTIFICATIONS OF INFLUENZA-LIKE DISEASES AND INFLUENZA VIRUS ISOLATIONS (BOXED) IN HONG KONG, 1962-68 March April [ May June July L Aug. Sept. J Oct. [ Nov. Dec. I Total 746 585 474 663 761 297 354 340 184 6 374 435 353 502 344 336 414 200 538 213 4433 263 232 239 322 408 [3 119 119 113 119 2 473 2 2el 81 83 106 20 35 28 28 30 896 497 a.4b44 27 44 1 27 19 16 25 17 1 220 10 17 51 38 99 1 705 a] 1 422 1 444 28 4 923 220 182 225 216 6214 1 478 196 283 179 106 8693 a Influenza A2 virus isolated. b Influenza B virus isolated. ,'6214 600 = 400 20 = 200 , l J F M A M J J A S 0 N D Month WHO 91428 350 NATIONAL INFLUENZA EXPERIENCE IN HONG KONG, 1968 FIG. 2 WEEKLY INCIDENCE OF INFLUENZA-LIKE DISEASES IN HONG KONG, JUNE TO SEPTEMBER 1968 15 22 29 6 June Jul, 13 20 27 3 10 Aug. Week ending 17 24 31 7 14 21 28 Sept. WHO 91427 its maximum intensity in the week of 27 July and gradually subsided in the following 3 weeks (Fig. 2). Altogether, the outbreak lasted for about 6 weeks. It was reported that the disease affected all age- groups and the clinical symptoms were considered mild, lasting for 3-5 days. There were no observable excess deaths during the epidemic. The data for the 1968 epidemic in Hong Kong are far from complete, because the figures supplied by 9 of the Government clinics represented only a small proportion of the affected people. Many attended private clinics or sought relieffrom Chinese herbalists. No useful information could be derived from ab- senteeism data for factories and schools; the majority of the labour force are workers who are paid daily and who do not report sick unless they have severe symptoms, and the schools were closed for the sum- mer vacation. However, it was suggested that about 15% of the population was affected in this epidemic. On 17 July, the laboratory isolated the virus strain in primary monkey kidney tissue culture and made a preliminary identification of subtype A2. Its hae- magglutinating activity was inhibited to only a low titre of 1:80 by the polyvalent A2 antiserum. The antiserum, which was supplied by the National Com- municable Disease Center in the USA, had a haemag- glutination-inhibiting titre of 1: 640 against the 1967 A2 strains. No dissimilarity of such magnitude had been observed in the previous A2 strains which were isolated between 1962 and 1967 in Hong Kong. Consequently, the virus strain was immediately dis- patched to the World Influenza Centre in London in the form of infected tissue culture. In the following week, 5 more lyophilized strains were sent to the World Influenza Centre and also to the International Influenza Center for the Americas in Atlanta, Ga. The strain was later confirmed to be a distinct anti- genic variant of A2 virus. Following a warning of its possible spread issued by the World Health Organization on 16 August, the virus was found to be causing epidemic outbreaks in other parts of the world in the later part of 1968. 3500 3000 _ 2500 _ cm .° 2000 to o. 15W0 1000 ~_ 500 0 351