UPDATE 1-Sanofi says under 10s may need two swine flu shots

* Clinical trial suggest one dose won't ensure protection

* Sanofi only H1N1 vaccine source for young children in U.S.

(Adds details, background)

LONDON, Oct 14 (Reuters) - Children under 10 years may need two shots of swine flu vaccine to get optimal protection, French drugmaker and the world's biggest flu vaccine producer Sanofi-Aventis (SASY.PA) said on Wednesday.

A two-dose regimen for H1N1 swine flu would be in line with recommendations for seasonal influenza immunisation in children of this age.

Sanofi said results of a U.S. clinical trial looking at H1N1 vaccination in children aged 6 months through 9 years suggested a single dose may protect many children but "two doses of vaccine will be required for optimal protection".

Previous results from studies of swine flu vaccines made by Sanofi and other companies have found one dose offers good protection in adults.

In the case of children, however, only 50 percent of those aged 6 to 35 months had adequate protection after a single shot, while 76 percent of children aged 3 to 9 years were protected. That compares with 98 percent of adults who had a good immune response after just one dose.

Sanofi is the only company licensed in the United States to produce a flu vaccine for children as young as 6 months of age.

A total of 474 children were studied in the clinical trial. (Reporting by Ben Hirschler; Editing by Hans Peters)