(Adds analyst comment)

By Toni Clarke

WASHINGTON, July 31 (Reuters) - The benefit of Baxter International Inc's experimental immune deficiency therapy, HyQvia, outweighs the risks, an advisory committee to the U.S. Food and Drug Administration concluded on Thursday.

The panel voted 15-1 that the available data shows a favorable benefit-to-risk ratio for the drug, which is designed to treat primary immunodeficiency diseases. These are genetic disorders in which certain cells of the immune system are missing, leading to infections, recurrent pneumonia and abscesses of the organs.

The FDA is not obliged to follow the advice of its advisory panels but typically does so.

The agency had raised concerns that some patients developed high levels of antibodies to a component of the treatment. Antibodies are used by the immune system to fight off viruses and other foreign substances in the blood.

Baxter's data showed the elevated antibodies had no clinical impact on patients. The FDA said there is, nonetheless, a possibility that long-term exposure to the product could cause inflammation of the brain and bowel, as well as fertility problems.

Panelists said the risk remains theoretical, and they voted against requiring routine monitoring of antibody levels. They also voted against restricting availability of the product. The FDA had asked whether certain subgroups of patients, such as pregnant women, male children and patients with certain inflammatory conditions, should be excluded.

The votes follow a far more cautious assessment of HyQvia by FDA reviewers on Tuesday.

Michael Weinstein, an analyst at J.P. Morgan, lowered his earnings estimate for Baxter on Thursday, before the panel meeting, to reflect the company's divestiture of its vaccines business and an assumption that HyQvia "fails to reach the US market following our negative read of the FDA panel documents."

Baxter's shares were down 1.3 percent at $75.08 in afternoon trading following the vote.

HyQvia, which was approved in Europe in 2013, combines immune globulin (IG), a substance made from human blood plasma, and recombinant human hyaluronidase, or rHuPH20, a genetically engineered enzyme that increases absorption of the IG and allows it to be used less frequently than traditional products.

The FDA declined to approve the product in 2012, and asked for more information about its possible impact on fertility after some patients in a clinical trial developed antibodies against rHuPH20, which Baxter licensed from Halozyme Therapeutics Inc.

Halozyme's shares rose 7.25 percent to $9.89 in afternoon trading following the vote.

"The risk-benefit considerations are very different for HyQvia compared to other IG products because the main immunogenetic component of concern (rHuPH20) is not a life-saving therapeutic," the FDA's report noted.

FDA scientists reiterated their concerns on Thursday, saying it could take years to fully understand the risks.

Existing IG therapies are given intravenously every three to four weeks in a hospital, or by injection at home every one to two weeks. HyQvia is designed to be injected at home every three to four weeks.

Derrick Sung, an analyst at Sanford Bernstein, said he believes HyQvia will be a "significant growth driver" for Baxter. He expects the product to be approved by the end of the year and estimates it could generate peak sales of $800 million.

(Reporting by Toni Clarke; Editing by Eric Beech and Leslie Adler)