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A drawing of a pig decorates the door of a lab where samples are tested for the H1N1 swine flu virus at the Maryland Department of Health and Mental Hygiene in Baltimore, September 3, 2009. REUTERS/Jonathan Ernst

H1N1 flu has killed at least 2,837 people but is not causing more severe illness than previously and the virus has not mutated, the World Health Organization (WHO) said on Friday.

The United Nations agency is monitoring the strain, commonly known as swine flu, to detect any mutation which might signal that it has become more deadly.

There is no sense that the virus has mutated or changed in any sense, WHO spokesman Gregory Hartl told a news briefing. We are continuing to see increased number of deaths because we are seeing many, many more cases.

About a quarter of a million cases have been laboratory-confirmed worldwide, but this is far fewer than the true number according to the WHO which has stopped requiring its 193 member states to report individual cases.

Its previous update of August 28 showed at least 2,185 deaths, meaning an additional 652 deaths were reported in the past week.

The virus could eventually infect 2 billion people, or a third of the world's population, according to WHO estimates.

In the best case scenario we have today, we will still have a moderate virus that is projected to cause several million deaths, Dr. Tammam Aloudat, senior health officer at the International Federation of Red Cross and Red Crescent Societies, told a separate news briefing.

Which means that even in the best case scenario, we do have an emergency on our hands, an emergency of a scale different from what we have seen before in the modern era, he said as the federation launched an information campaign to help the poorest communities reduce infection through simple hygiene measures.

H1N1, declared a pandemic on June 11, causes many mild symptoms, but has killed more people with medical conditions, such as asthma, or pregnant women, who should be treated with the antiviral Tamiflu by Roche Holding AG, the WHO says.

PREDOMINANT VIRUS

Many countries in South and Southeast Asia are reporting increasing or sustained high levels of respiratory disease, although Thailand has reported a declining trend, said the WHO's latest weekly update.

In Latin America, countries including Ecuador, Venezuela, Peru and Brazil are reporting more respiratory disease, while outbreaks seem to have peaked in Chile and Argentina, it said.

Japan is seeing an early start to its regular flu season. In Canada and the United States, influenza activity remains low overall, despite increases in the southeastern U.S. region.

Drugmakers are racing to develop vaccines amid experts' warnings that a second wave of the virus is approaching as weather cools in the northern hemisphere and the traditional flu season starts.

On Thursday, Novartis AG said a single dose of its vaccine might protect against the virus, raising hopes that potentially tight supplies could go further when mass immunization starts this month. 

In Latin America, countries including Ecuador, Venezuela, Peru and Brazil are reporting more respiratory disease, while outbreaks seem to have peaked in Chile and Argentina, it said.

Japan is seeing an early start to its regular flu season. In Canada and the United States, influenza activity remains low overall, despite increases in the southeastern U.S. region.

Drugmakers are racing to develop vaccines amid experts' warnings that a second wave of the virus is approaching as weather cools in the northern hemisphere and the traditional flu season starts.

On Thursday, Novartis AG said a single dose of its vaccine might protect against the virus, raising hopes that potentially tight supplies could go further when mass immunization starts this month.

Hartl has called the preliminary results encouraging, noting any vaccine that can be administered in one dose would substantially increase the number of overall doses available.