UPDATE 1-Norway says found H1N1 mutation in flu victims

* 3 H1N1 cases found with potentially significant mutation * Two cases found among first fatalities in Norway

* Says disease's mutation could cause more serious illness

* WHO says mutated virus sensitive to antivirals, vaccines

(Adds WHO statement in new paras 4-6; CDC comment paras 15-17)

OSLO, Nov 20 (Reuters) - Norwegian health authorities said on Friday they have discovered a potentially significant mutation in the H1N1 influenza strain that could be responsible for causing the severest symptoms among those infected.

"The mutation could be affecting the virus' ability to go deeper into the respiratory system, thus causing more serious illness," the Norwegian Institute of Public Health said in a statement.

There was no reason to believe the mutation had any implication for the effectiveness of flu vaccines or antiviral drugs made by groups such as Roche (ROG.VX: 行情), GlaxoSmithKline (GSK.L: 行情), Novartis (NOVN.VX: 行情) and AstraZeneca (AZN.L: 行情), the authorities said.

The World Health Organisation said that the mutation did not appear to be widespread in Norway and the virus in its mutated form remained sensitive to antivirals and pandemic vaccines.

A similar mutation had been detected in H1N1 viruses circulating in several other countries, including China and the United States, in severe as well as in some mild cases, it said.

"Although further investigation is under way, no evidence currently suggests that these mutations are leading to an unusual increase in the number of H1N1 infections or a greater number of severe or fatal cases," the WHO said in a statement.

H1N1, a mixture of swine, bird and human viruses, has killed at least 6,770 people globally, according to its latest update.

In Norway the mutation was found in the bodies of two people killed by the virus and of one person made seriously ill. The two infected by the mutated virus who died were among the first fatalities from the H1N1 pandemic in Norway, the institute said.

It was unclear whether the mutated virus was transmitted among humans, the health authorities said.

"Based on what we know so far, it doesn't seem like the mutated virus is circulating in the population, but rather that spontaneous changes have happened in the three patients," director Geir Stene Larsen at the public health institute said in the statement.

Norway has seen relatively more fatalities in the flu pandemic compared to the size of the population versus other European countries, with 23 confirmed deaths.

Public health authorities have said this could be due to the country being hit early in the pandemic's northern hemisphere winter wave, before a mass vaccination programme got underway.

"Nevertheless, it is important to study if there's still something about the Norwegian fatalities that separate us from other countries, and that make us learn something that strengthens our treatment of the seriously ill," director Bjorn-Inge Larsen at the Norwegian Directorate of Health said.

Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention said, "This mutation has been seen sporadically."

She said it is sometimes seen in patients who have mild influenza symptoms.

"I think it is just too soon to say what this might mean long term," Schuchat told reporters in a telephone briefing. (Reporting by Richard Solem; Additional reporting by Stephanie Nebehay in Geneva and Maggie Fox in Washington; Editing by Matthew Jones and Louise Ireland)