Such an outbreak, which would hit poultry and human beings, would probably take place as common flu cases reach their peak, said Zeng Guang, chief epidemiology scientist at China's Centre for Disease Control and Prevention.

Zeng said that the three major bird flu outbreaks over the past three years had all taken place during the winter or spring.

Great attention must be paid to the possible occurrence of common flu and bird flu peaks at the same time, Zeng told a forum jointly held by Sanofi Pasteur and the Chinese Preventive Medicine Association in Beijing.

It remains difficult to determine how the H5N1 virus will develop, said Zeng, but he noted there was a possibility it may form a hybrid with other flu viruses.

Possible crossbreeding may result in a new form of virus which could be transmitted between humans, he noted.

Currently, all human cases of bird flu have been infected by sick birds. There is no evidence to suggest that the virus can be passed from person to person, according to World Health Organization (WHO).

WHO experts have repeatedly warned there is a major risk that the bird flu virus may be transmitted between humans, which would result in a global pandemic.

Zeng said that the three flu pandemics over the past century were all caused by a hybrid virus.

Flu pandemics generally take place three or four times per century.

The last major pandemic took place in 1918-1919 and killed an estimated 40-50 million people across the globe.

Flu viruses can be divided into three groups A, B and C. Only A, which infects many animal species such as birds and swine, and B, which only affects humans, can cause severe disease and lead to epidemics.

Bird flu is an infectious disease caused by A viruses.

Highly pathogenic bird flu, such as that caused by the H5N1 strain currently circulating in Asia and other parts of the world, is characterized by its sudden onset, severe illness, and generally quick death.

The H5N1 strain is of particular concern because it mutates rapidly and can acquire genes from viruses infecting other animal species. This highly pathogenic strain is now known to cause severe disease and death in humans.

As the disease increases among birds and humans, the likelihood also increases that a human concurrently infected with human and bird flu strains will serve as the "mixing vessel" for a new influenza subtype that can be transmitted easily from person to person, thus sparking a flu pandemic, according to the WHO.

At present, there remains one prerequisite for the start of a pandemic that the current H5N1 virus has yet to meet sustained and efficient transmission among humans.

Currently, about 250,000 to 300,000 people die across the globe every year as a result of complications caused by various types of influenza, he said.

In this regard, it is vital to prevent common influenza in the war against a possible bird flu epidemic or pandemic, he added.

In China, around 130 million people are infected with influenza every year.

From November 2003 to September 14 this year, a total of 246 people had been infected by H5N1 in 10 countries, resulting in 144 fatalities. A total of 21 people in China have been infected by the virus, with 14 losing their lives.

Evidence to date indicates that close contact with sick or dead birds, such as slaughtering or de-feathering, is the principal source of infection.

Worldwide, about a dozen companies are currently conducting clinical trials on bird flu vaccines.

As scientists still do not know enough about the virus, public education is an important way to prevent a bird flu outbreak, said Zhang Bin, an official from the Ministry of Health.