The swine flu virus hasn't mutated into a more deadly strain but there are early signs it is developing resistance to vaccine, the World Heath Organization's chief said.

Authorities are monitoring closely whether the virus was morphing into more virulent forms that would make it deadlier, the organization's Director-General Margaret Chan said.

"We are not seeing that situation right now," Chan told reporters as the WHO convened a conference in Hong Kong.

The WHO says the swine flu virus _ also known as H1N1 _ has killed almost 3,486 people worldwide as of Aug. 13. South America and North America account for the majority of deaths.

For now, the infection is generally mild and most people recover without treatment. But should it become deadlier, developing nations could be especially vulnerable because those populations lack adequate health care and are already fighting a myriad of diseases including AIDS, malaria and tuberculosis.

Chan said manufacturers were on track to develop billions of new doses of the vaccine over the next year. The vaccine is highly effective against the swine flu virus, though there were a small number of instances _ about 25 in the world _ of a vaccine-resistant flu.

Sanofi-Aventis SA will begin delivering the first doses of its new swine flu vaccine in the United States by mid-October, the head of France's largest pharmaceutical company said.

In an interview with French daily Le Figaro, Chris Viehbacher said deliveries of the vaccine in France could begin by late November, after approval by European drug regulators.

Viehbacher said Sanofi-Aventis will be able to produce at least 800 million doses of the vaccine per year.

Last week the US Food and Drug Administration approved the new swine flu vaccine, a long-anticipated step as the US government works to start mass vaccinations next month.

The vaccine is being made by CSL Ltd. of Australia, Switzerland's Novartis Vaccines, Maryland-based MedImmune LLC and Sanofi Pasteur of France _ which produces flu shots at its Swiftwater, Pennsylvania, factory.

London-based GlaxoSmithKline also was expected to supply vaccine.

The US has ordered 195 million doses but may order more if there's enough demand. Typically fewer than 100 million Americans seek flu vaccine every year, and it's unclear whether swine flu _ what scientists prefer to call the 2009 H1N1 strain _ will prompt much more demand.