UPDATE 1-Baxter says ramps up swine flu vaccine yields

* 3-fold increase in production yields

* Says unclear if Baxter vaccine protects with one dose

CHICAGO, Sept 16 (Reuters) - Baxter International Inc (BAX.N) on Wednesday said its production yields of a "swine flu" vaccine have increased three-fold.

In a presentation to investors, Joy Amundson, president of Baxter's Bioscience unit, said the company has begun shipping the swine flu, or H1N1, vaccines weekly.

Vaccine makers have been struggling with yields, creating concerns of a shortfall.

The new H1N1 strain of flu, declared a pandemic on June 11, could eventually infect 2 billion people, according to World Health Organisation (WHO) estimates.

Novartis said earlier this month a single dose of its swine flu vaccine might protect against the virus, raising hopes that potentially tight supplies could go further when mass immunisation starts.

A Baxter spokeswoman said it still was not known whether the Baxter product could give protection with a single dose.

She said it was also unclear whether or not Baxter would be able to fulfill its vaccine orders. But reports that it would not be able to meet its commitments were premature, she said.

It depends on yields, she said, and they are improving.