July 10, 2009: Jason Gale (Bloomberg)—An unexpected characteristic has emerged among many swine flu victims who become severely ill: They are fat.

Doctors tracking the pandemic say they see a pattern in hospital reports from Glasgow to Melbourne and from Santiago to New York. People infected with the bug who have a body mass index greater than 40, deemed "morbidly obese," suffer respiratory complications that are harder to treat and can be fatal.

With the new virus on a collision course with the obesity epidemic, the World Health Organization says it’s gathering statistics to confirm and understand this development. Drugmaker Roche Holding AG is combing through studies to determine whether heavier people should get bigger doses of its Tamiflu antiviral.

"Morbid obesity is one of the most common findings turning up in severely ill patients," said Nikki Shindo, who is leading the investigation of swine flu patients at the WHO in Geneva. "It’s a huge problem."

In Canada’s Manitoba province, three out of five people treated for the new flu strain in intensive care units are obese, said Ethan Rubenstein, head of infectious diseases at the University of Manitoba in Winnipeg. Patients with flu symptoms should be considered at risk of complications if they carry excess weight, according to Rubenstein.