PARIS (Reuters) - Sanofi Pasteur's early stage trials of an experimental vaccine against H5N1 bird flu shows it provides a good immune response in humans and is safe, the French company said on Thursday. 

Sanofi Pasteur, the vaccine unit of drug maker Sanofi-Aventis, announced the "encouraging" preliminary results of the Phase I trial among 300 healthy volunteers in France. 

Governments around the world are attempting to halt a growing outbreak of H5N1 bird flu and prevent an influenza pandemic from developing. The virus has killed 71 people out of 138 known human cases, all of them in Asia. 

Signficantly, the drug maker said its vaccine proved effective when given at a lower dose than in an earlier trial, which would allow it to give the shot to more people in the event of a pandemic triggered by bird flu. 

A 30 microgram dose with an adjuvant -- an additive that boosts the immune response -- in a two-dose regimen showed an immune response consistent with the requirements of regulators to approve seasonal influenza vaccine, Sanofi Pasteur said. 

The company plans to conduct further Phase II trials in 2006, a company official said. 

The good immune response, Sanofi Pasteur said, was seen in a significant number of volunteers who tolerated the vaccine well, laying the basis for further trials and research. 

"Sanofi Pasteur characterised the preliminary results as a sign of progress and providing direction for further development of a pandemic influenza vaccine," it said in a statement. 

Immune responses were also seen in a number of volunteers receiving lower doses. Further trials will examine different dosages that could help low-dose strategies widely discussed by public health experts. 

"MOCK UP" DOSSIER 

Experts fear the H5N1 virus could mutate into a form easily transmitted from person to person and sweep the world, killing millions within weeks or months. So far, almost all human cases can be traced to direct or indirect contact with infected birds. 

The French group is among several vaccine producers, including GlaxoSmithKline Plc and Chiron Corp, racing to develop a vaccine to protect humans from bird flu. 

No one knows how well an H5N1 vaccine will match a future pandemic strain but it could "prime" a person's immune system so they will get stronger effects from a later, better-matched vaccine. 

"It's good that they seem to be ready to go, to move ahead," said WestLB analyst Oliver Kaemmerer.  

He noted, however, that the current "pre-pandemic" shot was only a halfway house in developing a truly effective pandemic vaccine, which can only be produced once a new strain of flu that spreads easily from humans to humans emerges. 

Sanofi has already agreed to create a 1.4 million dose stockpile of pre-pandemic vaccine for the French government and to supply enough vaccine to protect up to 28 million people in the event of a pandemic being declared.