swinefluLONDON — French drugmaker Sanofi Pasteur began human trials of its swine flu vaccine this week in about 2,000 people in the United States, the company said Friday — becoming the third company conducting clinical trials on swine flu vaccines.

Sanofi Pasteur’s trials began Thursday, testing both the vaccine’s safety and how many doses would be needed to protect people from swine flu, the company said in a statement. Most experts think two doses will be necessary.

The World Health Organization’s vaccine chief Marie-Paule Kieny said Thursday that drugmakers were “on track,” and that the first batches of swine flu, or H1N1, vaccine should be delivered in September.

Sanofi Pasteur, which makes about 40 percent of the world’s flu vaccine, has applied to the U.S. Food and Drug Administration for fast-track licensing for the vaccine.

Kieny said safety was not compromised in granting quick licenses before full safety tests are finished — a practice European and U.S. drug regulators also use for seasonal flu vaccines. Clinical tests on new vaccines can take up to a year or more.

The European Medicines Agency said approving a swine flu vaccine for EU countries could take five days.

The WHO said Northern Hemisphere countries, as well as Southern Hemisphere doctors and nurses, should get a vaccine first because swine flu is expected to surge in the north when the regular flu season begins later this year.

Sanofi Pasteur has contracts with several governments, including France and the U.S., to provide swine flu vaccine and other products in bulk.

Meanwhile, Swiss pharmaceutical Novartis AG said this week it started testing its swine flu vaccine in about 6,000 people in Britain, Germany and the U.S.

Last month, Australian company CSL also began testing its vaccine in Australia.

The vaccines being tested by Novartis and Sanofi Pasteur contain adjuvants — ingredients intended to boost the body’s immune response. Adjuvants, which also stretch a vaccine’s active ingredient further to make more doses, are used in European flu vaccines but not in the U.S. The WHO has recommended they be used in swine flu vaccines to increase the global supply.

Health officials have said there is little or no information on how vaccines containing adjuvants affect pregnant women and children — two groups thought to be particularly vulnerable to swine flu.