Pandemic flu harder to survive in poor nations: WHO

* Patients developing severe disease need costly care

* Nearly certain "devastating impact" in poor countries

By Laura MacInnis

GENEVA (Reuters) - H1N1 influenza is "a virus of extremes" likely to cause far more deaths in poor countries than affluent ones, the head of the World Health Organization said on Tuesday.

WHO Director-General Margaret Chan said the pandemic flu virus caused mild symptoms in most patients, the overwhelming majority of whom recover fully within a week and without medical treatment.

But she warned the small subset of patients who developed serious illness from the strain widely known as "swine flu" needed very specialized, intensive care to survive infection.

"Clinically, this is a virus of extremes. It does not seem to have a middle ground," she told a WHO regional meeting in Copenhagen, according to the text of her remarks.

Pregnant women, diabetics and those with stressed immune systems have been most vulnerable to severe infection from H1N1, which was discovered in North America in April and declared a global pandemic in June.

Chan said that such patients "will be in grave danger" in countries lacking adequate hospitals, doctors, nurses, antivirals, vaccines, and clean water.

"The same virus that causes manageable disruption in affluent countries will almost certainly have a devastating impact in countries with too few health facilities and staff, no regular supplies of essential medicines, little diagnostic and laboratory capacity, and vast populations with no access to safe water and sanitation," she said.

"For these populations, advice such as wash your hands, call your doctor, or rush to the emergency ward will mean very little," the former Hong Kong health director stressed.

Such guidance has dominated the official H1N1 response in wealthy nations, where authorities have been watching for severe infections and contemplating school closures as a way to slow the transmission of the virus.

The WHO has estimated that 2 billion people could eventually catch the new strain and governments worldwide are scrambling to secure access to vaccines under development by pharmaceutical companies including Novartis, GlaxoSmithKline, Sanofi-Aventis and Solvay.

(Editing by Stephanie Nebehay)