* U.N. agency changes recommendations for treating flu

* Antivirals urged for sick pregnant women, young children

* Can avert severe pneumonia and deadly complications

(Recasts with details of changed recommendations, fresh quotes)

By Laura MacInnis

GENEVA, Nov 12 (Reuters) - Antiviral medicines can prevent H1N1 flu deaths and should be given quickly to pregnant women, very young children and people with underlying medical problems who fall ill, the World Health Organisation said on Thursday.

Announcing a change of its guidance to doctors, the United Nations health agency said the drugs should be administered even before tests conclude that an at-risk patient has the pandemic virus and not something else.

"We have updated our clinical guidance to emphasise that seeking early medical attention can save lives," Nikki Shindo of the WHO's global influenza programme told journalists on a teleconference.

The H1N1 strain, which the WHO declared a global pandemic in June, can cause severe pneumonia in previously healthy people. It has killed more than 6,000 people and spread to 199 countries since its discovery in North America earlier this year.

Pregnant women, children under the age of 2 and people with respiratory problems and other diseases are at highest risk of the extreme effects of swine flu, which can take hold as soon as one week after infection by the highly contagious virus.

"The window of opportunity is very narrow to reverse the progression of the disease," Shindo said. "The medicine needs to be administered before the virus destroys the lungs."

The new clinical guidelines also recommend that people outside the at-risk group who have "persistent or rapidly worsening symptoms," such as problems breathing or a high fever for more than three days, should take antivirals.

These should be administered by a doctor, and are not needed by people who have only a typical cold, Shindo stressed.

TAMIFLU, GENERICS

Tamiflu, an antiviral marketed by Switzerland's Roche Holding <ROG.VS> and known generically as oseltamivir, is the frontline drug recommended by the WHO as a way to treat and slow the progression of flu symptoms.

Other companies including India's Cipla Ltd <CIPL.BO> make generic versions of Tamiflu. The H1N1 virus has not shown much resistance to the drug partly because the new strain has not mutated or changed as it has spread around the world. [ID:nNWLA7419]

Shindo said the virus was "amazingly stable," reflecting the fact that it can easily breach the immune systems of people who lack natural defences to the strain, which had never been seen before its emergence in Mexico and the United States.

Many hospitals and clinics, especially in poorer countries, have been overwhelmed with patients seeking care for H1N1 as the northern hemisphere has entered its winter flu season.

Shindo said early treatment of at-risk and severe patients with oseltamivir could help ease that strain.

The WHO shipped antiviral drugs to 72 countries in May as the pandemic began to gain speed, and recently sent stocks of the medicines to Afghanistan, Mongolia, Belarus and Ukraine.

It will soon send more supplies to Azerbaijan and Kyrgyzstan. (Additional reporting by Stephanie Nebehay; editing by Robin Pomeroy)

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