The first vaccines to combat H1N1 swine flu should be approved and ready for use in some countries from September, the World Health Organization said on Thursday.

Marie-Paule Kieny, WHO director of the Initiative for Vaccine Research, said manufacturers had initially reported poor yields in making vaccine, leading to worries about supplies, but this was now improving.

We are on track in development, Kieny told reporters.

First results from clinical trials are expected early next month and these tests will show whether one or two doses are needed to provide immunity -- another big swing factor in determining how many people can be vaccinated.

Once initial clinical trial results are in, regulators will be able to approve the vaccines from next month, Kieny added.

The H1N1 flu outbreak, declared a pandemic on June 11, has spread around the world and could eventually affect 2 billion people, according to WHO estimates.

Fears the strain could become resistant to the anti-viral drug Tamiflu have underscored the need to get vaccines to market quickly. The H1N1 vaccines would be given separately from regular seasonal flu shots.

Leading flu vaccine makers include Sanofi-Aventis, GlaxoSmithKline, Novartis, Baxter, CSL and Solvay.

WHO Director-General Margaret Chan has said the H1N1 virus is stable and there were no signs of it mixing with other more dangerous types of influenza such as the deadly H5N1 bird flu strain.

At present, patients with mild symptoms generally do not need any medicines to recover, and Chan has stressed hospital visits are not necessary unless those infected with flu have certain warning signs.

These include long-lasting high fever in adults and a lack of alertness in children. Pregnant women and people with health problems including diabetes are also vulnerable to more severe effects from the new flu strain.

Medicine