In May 1997 influenza A subtype H5N1 virus was isolated in the Special Administrative Region of Hong Kong from a child who died. Until then, the H5N1 virus was known to infect only various species of birds, including chickens and ducks. After the first human case, intensive surveillance revealed 17 additional cases by the end of 1997, all of them in Hong Kong. Six deaths occurred. To date (13.02.98), no further case has been detected, the last one having been detected on 28 December 1997.

Human infection with influenza virus may be asymptomatic, and causes a spectrum of illness whose severity ranges from mild to fatal. Elderly people and young children tend to have more severe illness in general, but symptoms associated with infection with H5N1 have been severe in other age groups as well. The severity of infections identified so far may not be representative of the spectrum of illness caused by H5N1 infection in humans, however, as milder cases may not have been identified.

The response of the international health community was organised through several main axes:
- to strengthen surveillance activities in humans and birds.
- to determine the characteristics of viruses isolated from the 18 confirmed H5N1 cases.
- to determine how these people became infected.
- to establish whether other people have been infected by influenza H5N1 virus in Hong Kong and southern China.
- to collect evidence of possible human to human transmission.
- to prepare seed viruses for setting up a H5N1 vaccine.

All the viruses isolated from the eight first cases showed genetic characteristics of the avian virus of H5N1 type. Influenza A viruses are known to mutate particularly easily. A further mutation could lead to the emergence of an H5N1 strain that is more transmissible.

Most of the surveillance activities was focused on hospitals, but surveillance was also reinforced in outpatient facilities in Hong Kong and China and has been extended to all health centres in Hong Kong since 8 December 1997. The control of the production and transport of pigs and poultry to be sold in markets was intensified in Hong Kong and in neighbouring areas of China.

The World Health Organization (WHO) collaborating centre for Studies in the Ecology of Influenza in Animals at St Jude's Research Hospital in Memphis (Tennessee, United States) set up an intensive epidemiological surveillance of birds and other animals in Hong Kong in close collaboration with the Hong Kong authorities and the Hong Kong University. The aim is to identify the natural reservoir of the virus and to assess its spread in animal populations. Chickens are not considered to be an efficient reservoir of the virus, as infection causes fatal disease in this species. More than 2000 samples were taken from 12 bird and animal species. Preliminary results identified the H5N1 virus in 10 cultures from domestic and wild ducks and in wild geese collected in Hong Kong markets before about 1.6 million chickens and other birds were slaughtered early in January. The virus had previously been isolated only in chickens. These results have not enabled the initial source of the H5N1 virus to be identified. Neither do they show whether the domestic ducks raised on Hong Kong farms have been infected with the virus.

A team of experts from the WHO collaborating centre at the Centers for Disease Control and Prevention (CDC) in Atlanta, in conjunction with the WHO collaborating centre at the National Institute of Infectious Diseases in Tokyo, carried out a detailed investigation to help the influenza centre and the department of public health in Hong Kong to estimate the importance of these findings and their impact on public health. Blood samples were taken from relations of infected patients, health care staff, other contacts, and people with no risk factor for exposure. Preliminary results showed that less than 1% of people who had potentially been in contact with the first case had specific antibodies in their serum. A practitioner who had treated the patient was infected but no transmission was identified between family members. The exact mode of transmission of H5N1 to humans has not yet been identified but infection with the virus is believed to be associated with infected living birds. There is no clear evidence of human to human transmission. Serological trials have confirmed that human to human transmission is “relatively ineffective”. Risk of transmission of the H5N1 virus to humans from raw, chilled or frozen poultry foods has not been demonstrated. Although H5N1 is unlikely to be transmitted through consumption of foods, the application of WHO's 'Ten Golden Rules for Safe Food Preparation' is recommended to provide adequate protection from all poultryborne diseases. In particular, attention should be given to the thorough cooking of foods, avoiding contact between raw and cooked foods, and diligent handwashing during food preparation.

WHO does not currently recommend that a specific H5N1 vaccine should be produced but WHO collaborating centres on influenza are working on various H5 strains to prepare high growth reassortants, which could be used for vaccine production in case of need. The preparation of a vaccine would take several months after the selection of a suitable virus strain.

In the meantime, the WHO collaborating centre for influenza at CDC in Atlanta has prepared a kit of reagents for diagnosis of H5N1 infection, which has been dispatched to the 110 national influenza centres in 82 countries that make up the WHO network for influenza surveillance.

In the absence of any sign of human to human transmission of influenza H5N1 virus, no special measures - such as travel restrictions or quarantine - are justified in Hong Kong or elsewhere, according to WHO.

Two years ago, WHO created a Task Force of Experts on Influenza. This special task force is currently developing a draft plan for the global management and control of influenza pandemics. Elements of this plan include augmented surveillance for and identification of potential pandemic viruses, dissemination of information, logistic and other support to national health authorities, the promotion of high growth seed virus for vaccine, and the facilitation of vaccine production and its international distribution. Moreover, the plan calls for each national authority to develop its own pandemic emergency response plan.