By Maggie Fox, Health and Science Editor

WASHINGTON, Sept 3 (Reuters) - GlaxoSmithKline <GSK.L> released a new plan on Wednesday for companies to stockpile its influenza drug Relenza in case of a pandemic.

The plan gives businesses two options: to buy Relenza now at a discount with free storage, or to pay an annual fee to reserve the flu drug for future purchase at a capped rate.

"We are committed to helping employers and other business leaders prepare for an influenza pandemic and its impact on the health of their employees and the operational integrity of their organizations," Chris Viehbacher, president for North American pharmaceuticals at the company, said in a statement.

In June, rival Roche AG <ROG.VX>, which makes the flu drug Tamiflu, offered a similar plan.

Most experts agree that a pandemic of some type of influenza is certain, although no one can predict when or what strain might strike.

The main suspect is H5N1 avian influenza, which has swept through flocks of birds in Asia, Europe, the Middle East and Africa and become entrenched in ways that experts say has never happened before.

It rarely infects people but has killed 243 out of 385 people infected globally so far, according to the World Health Organization.

The WHO has recommended that countries prepare for such a pandemic, and the U.S. Health and Human Services Department has also encouraged employers to do so. Many companies are developing plans that include treating employees during a pandemic.

Several experimental vaccines are being tested against H5N1, but if a pandemic were to start, it would likely be months before vaccine production could begin to be ramped up.

Relenza, known generically as zanamivir, and Tamiflu, known generically as oseltamivir, are both used to treat seasonal influenza. Tamiflu is the drug of choice for H5N1 infection although experiments suggest Relenza would work, too.

Last month GlaxoSmithKline and its Australian partner Biota Holdings Ltd <BTA.AX> settled a damages claim over Glaxo's marketing of Relenza. Glaxo recently has started to ramp up Relenza production and sales.

Roche licenses Tamiflu from California-based Gilead Sciences <GILD.O>.

Tamiflu, a pill, has far outsold Relenza, which is an inhaled powder. But governments recommend stockpiling both.

A new influenza drug, peramivir, is being developed by Biocryst Pharmaceuticals Inc <BCRX.O> but it must be injected and it has not performed well in clinical trials.

Two older drugs are available but flu viruses have quickly developed resistance to them, although some experts believe they may be useful in cocktails with newer drugs.

Flu viruses have a high rate of mutation, which allows them to adapt to the treatments devised to tackle them, so experts recommend having a variety of drugs on hand to treat patients.

"A diversified stockpile of antivirals is important as we are coming to understand that our concerns for the past few years about resistance of pandemic influenza viruses to antivirals were justified," said Dr. Anne Moscona of New York-Presbyterian Hospital/Weill Cornell Medical Center. (Editing by Will Dunham)

AlertNet news is provided by