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UPDATE 1-Sinovac says one-shot swine flu vaccine effective

* Chinese group first to complete H1N1 vaccine trial

* Single shot means more people can be vaccinated

* Sinovac shares up 10 pct

(Writes through, adds detail, shares, background)

LONDON, Aug 18 (Reuters) - An H1N1 swine flu vaccine developed by China's Sinovac Biotech SVA.A is effective after just one shot, its developer said on Tuesday.

The news boosted shares in the Chinese vaccine company by 10 percent in U.S. trading.

Until now, experts have predicted that two shots will be needed to provide swine flu immunity. But Sinovac, which is the first company worldwide to complete clinical trials for an H1N1 vaccine, said a single dose of its vaccine proved sufficient.

Leading flu vaccine makers including Sanofi-Aventis SASY.PA, GlaxoSmithKline GSK.L, Novartis NOVN.VX, Baxter BAX.N, CSL CSL.AX and Solvay SOLB.BR are racing to develop a vaccine against swine flu.

The H1N1 flu outbreak, which was declared a pandemic on June 11, has spread around the world and could eventually affect 2 billion people, according to the World Health Organisation.

Officials are concerned that supplies of vaccine will prove tight, but an ability to vaccinate people once instead of twice would stretch stocks substantially.

Sinovac said immunogenicity after one dose of its vaccine had “reached the international criteria for vaccines”. There were no signs of severe adverse reactions, it added.

Its clinical trials were initiated in Beijing in July and inoculation was completed on Aug. 15, with a total of 1,614 participants over three years receiving the vaccine.

Regulators around the world are expected to start approving swine flu vaccines next month as more clinical trial results come in, allowing governments to start mass vaccination programmes from September, according to the WHO.

Medical experts say vaccines need to available quickly and in large quantities to have the greatest impact.

Vaccines arrived too late in the 1957 and 1968 flu pandemics to be of much use and flu vaccines had not been developed in the 1918 “Spanish flu” pandemic which killed an estimated 50 million. (Reporting by Ben Hirschler; Editing by David Holmes)