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 the winter and spring.

The vaccines have been available since mid September, when an inoculation campaign began, said a statement by the China Preventive Medicine Association. Twenty percent more doses are available compared to last year.

Six million to 7 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 US organizations have recommended minimal use of the preservative in production.

About 3 million doses of flu vaccine will be provided for babies below 3 years old this year, nearly 30 percent more than last year, according to the company.

The global H5N1 outbreaks since 2003 all occurred during the 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, who quoted epidemiological research results released by the World Health Organization in June.

Experts are concerned about the possibility that at some point, the bird flu virus could mix with human flu virus, making the combination virus potentially for more deadly and transmissible than either virus on its own. And that would create the potential for 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 localized outbreaks always happen, such as those in schools in the first quarter this year.

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

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

A bird flu vaccine developed by China for human was safe in preliminary clinical tests, the government reported in August.

So far, it has infected 247 people and killed 144 throughout the world as of September 19, according to WHO figures.