BEIJING -- Chinese health officials are preparing 23 million to 25 million doses of flu vaccine, after experts warned the H5N1 bird flu could break out again with other flu strains during winter and spring.

The vaccines would be available from mid September, when an inoculation campaign began, said a statement by the China Preventive Medicine Association. The quantity of doses is 20 percent more than last year.

Six million to seven million doses will contain no preservative, about eight times as many as last year, according to Sanofi Pasteur, the major provider of flu vaccines to China.

The preservative in the other vaccines was a mercuric compound used to prevent contamination in production and transportation. It has proved harmless so far, but some U.S. organizations have recommended minimal use of the preservative in production.

About three million doses of flu vaccine will be provided for babies below three years old this year, nearly 30 percent more than last year, according to the company.

The H5N1 outbreaks since 2003 around the globe all happened in winter and spring, so it was highly possible another outbreak might occur if the trend continued. It coincided with the high prevalence season for human flu, said an expert quoting epidemiological research results released by the World Health Organization (WHO) in June.

It was difficult to predict future mutation of the bird flu virus, but one possibility was that the virus mixed with human flu viruses, creating a new type which could trigger a human pandemic, said Zeng Guang, an expert with the Chinese Center for Disease Control and Prevention.

Zeng said no major flu outbreaks had occurred in China in the past few years, but localised outbreaks happened constantly, including some in schools, in the first quarter of this year.

Figures from the Ministry of Health showed a 13 percent rise in the flu incidence rate in the first quarter compared with the same period of last year.

As human flu outbreaks always occurred suddenly and were hard to predict, it was crucial to take preventive measures, especially when the H5N1 strain was still occurring, he said.

China has reported 21 human infections of bird flu since 2003, including 14 deaths.

The country's bird flu vaccine for human use had been proved safe and effective in preliminary clinical tests, announced the government in August.

Experts fear the H5N1 bird flu virus could mutate into a form able to spread among people, causing a global pandemic. It had infected 247 people and killed 144 throughout the world by September 19, according to WHO figures.