U.S. researchers reported on Friday that people who are obese, despite being otherwise healthy may be at a special risk of complications and death from the current H1N1 swine flu virus.

The connection between obesity and susceptibility to swine flu is being made as a high proportion of people who have fallen severely ill from swine flu have been found to be obese or severely obese.

Although obesity alone has not been seen as a risk factor for swine flu, health officials feel this could be due to the fact that many obese people tend to suffer from asthma and other such conditions which increase their susceptibility.

Health officials detailed the case of 10 people from Michigan with swine flu who were hospitalized in Ann Arbor and had to be put on ventilators. Three died and nine of the ten fell in the category of obese seven extremely obese while only three of the ten had any other health problem. Of the three people who died two of them had no other health conditions. 

Five of the patients developed blood clots in their lungs, and six had kidney failure, complications that have been seen in some swine flu patients before, but not usually in such a high proportion.

Dr. Tim Uyeki, a flu expert at the Centers for Disease Control and Prevention and a co-author of the report said, "Clinicians need to be aware that severe complications can occur in patients with the novel H1N1 virus, particularly in extremely obese patients." 

The study was published in advance in the Centers for Disease Control and Prevention's Morbidity and Mortality Weekly Report.

Dr. Lena Napolitano of the University of Michigan Medical Center and colleagues studied the cases of 10 patients admitted to the university's intensive care unit. In their report they said, "Of the 10 patients, nine were obese (body mass index more than 30), including seven who were extremely obese (BMI more than 40)."

Nine had multiple organ failure, which can be seen in influenza, but five had blood clots in the lungs, and six had kidney failure. The researchers said none has fully recovered.

Uyeki said, "What this suggests is that there can be severe complications associated with this virus infection, especially in severely obese patients.

“And five of these patients had ... evidence of blood clots in the lungs. This has not been previously known to occur in patients with severe influenza virus infections," Uyeki added.

The CDC's commentary accompanying the report said, "The high prevalence of obesity in this case series is striking."

"Whether obesity is an independent risk factor for severe complications of novel influenza A (H1N1) virus infection is unknown. Obesity has not been identified previously as a risk factor for severe complications of seasonal influenza."

Tamiflu is the treatment for swine flu and Uyeki said patients seem to survive better if they get Tamiflu for longer than the usual five-day treatment course. "We don't know if it is necessary for a higher dose of the drug to be given to patients who are obese," he said.