WASHINGTON - A DEFINING characteristic has emerged among many Influenza A(H1N1) victims who become severely ill: They are extremely overweight.

Experts tracking the pandemic report that patients with a body mass index greater than 40 - deemed morbidly obese - suffer respiratory complications that are harder to treat and can be fatal.

The World Health Organisation (WHO) is gathering statistics to confirm and understand this development.

And drugmaker Roche is checking whether heavier people need bigger doses of its Tamiflu antiviral.

'Morbid obesity is one of the most common findings turning up in severely ill patients,' said Dr Nikki Shindo, the WHO's lead investigator. 'It's a huge problem.'

Scotland, with the highest obesity rate in Europe, reported the continent's first two deaths from H1N1 and has chalked up a fifth of the region's fatalities.

In Canada's Manitoba province, three out of five H1N1 patients treated in intensive care units are obese.

But over in Japan, where only 1.6 per cent of adults are obese, there have been no deaths or severely ill H1N1 patients among the 2,146 confirmed cases.

Dr Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases in the United States, said: 'About 75 per cent of patients have underlying conditions and, clearly, obesity stands out as a statistically significant factor involved in the seriousness of the disease.'

Still, this is the first time that obesity has been noticed among severely ill flu sufferers on this scale, he said.

The H1N1 bug has killed about 440 people and infected 100,000 worldwide since April. Most of those who died were pregnant women or people with asthma, diabetes or other chronic diseases.

Read the full story in Saturday's edition of The Straits Times

BLOOMBERG