Website of the Department of Health

2.2.1 A/H5N1: The current situation

The current outbreak of highly pathogenic avian flu in Asia known as A/H5N1 - unprecedented in its scale and rate of spread - is thought to have significantly heightened the risk of another flu pandemic. Since its emergence in poultry in Korea in mid-December 2003, this strain has infected poultry in nine countries in Asia (including Hong Kong) (see Figure 2.1). There have also been, to date, 55 reported cases of human infection with the virus of which 42 have been fatal. Three countries have reported human cases of A/H5N1 infection: Cambodia, Thailand and Viet Nam - countries with widespread outbreaks in poultry.

Although A/H5N1 has appeared before, infecting 18 people and killing six of them in Hong Kong in 1997, the current epidemic of this strain represents a more serious threat to public health. This time it has caused outbreaks at the same time in several countries and is proving difficult to eliminate.

Transmission to people remains relatively rare and in most cases, investigation has identified contact with infected poultry as the principal cause of infection. Although human infections have been documented in only three countries, it seems likely that additional cases have occurred in other countries but have remained unrecognised because of a lack of clinical awareness or diagnostic facilities.

A chronology of the current outbreak of A/H5N1 from January 2004 is available on the World Health Organization website:

• WHO: H5N1 avian influenza: a chronology of key events (opens new window)

Table 2.2 Cumulative number of confirmed human cases of avian influenza H5N1 between 28 January 2004 and 10 February 2005.

To keep up to date with the number of human cases of A/H5N1, visit the website of the World Health Organization's Communicable Disease, Surveillance and Response unit at:

• World Health Organisation (opens new window)

2.2.2 How could A/H5N1 cause a pandemic in people?

Experts fear that A/H5N1 could trigger the next pandemic for several reasons. Firstly, it has already demonstrated an ability to infect people and cause severe disease - one of the key characteristics of a pandemic strain. Secondly, this particular virus has a documented ability to mutate and to acquire genes from viruses infecting other species. Experts fear that the virus could, either adapt, giving it greater affinity for humans, or exchange genes with a human flu virus, thereby producing a completely novel virus capable of spreading easily between people, and causing a pandemic.

The continued spread of A/H5N1 in birds increases the opportunities for direct infection of people. If more people become infected over time, the likelihood also increases that, if they are concurrently infected with human and avian influenza strains, they could serve as the 'mixing vessel' for the emergence of a novel subtype virus with sufficient human genes to be easily transmitted from person to person. Such an event could mark the start of an influenza pandemic. However, the likelihood of this mutation occurring is not easy to predict.

Person-to-person transmission

Person-to-person transmission

Human infection with A/H5N1 has been rare up to now. The virus has not acquired the ability to pass easily from person to person. Should it acquire this characteristic, it would meet all the criteria of a pandemic flu strain.

While there have been instances of possible person-to-person transmission, so far these have been one-off, isolated occurrences. Person-to-person transmission must be efficient and sustainable if the virus is to become capable of causing a pandemic. In other words, there must be a sustained chain of transmission causing community-wide outbreaks. So far, despite intensified surveillance, there is no strong evidence of this occurring.

Why is A/H5N1 difficult to eradicate in poultry?

Why is A/H5N1 difficult to eradicate in poultry?

The eradication of A/H5N1 in domestic birds is of major importance in pandemic influenza prevention. However, despite international efforts, A/H5N1 has so far escaped elimination. There are several reasons for this:

For these reasons, elimination of highly pathogenic avian flu in Asia is expected to take several years (and may not even be achievable), during which time the possibility that the virus could mutate into a pandemic strain remains.