(Corrects Tamiflu reference in paragraph 14 -- usual course is five days, not 10 days, and makes clear dose is normal, not extra-high)

WASHINGTON (Reuters) - People who are obese but otherwise healthy may be at special risk of severe complications and death from the new H1N1 swine flu virus, U.S. researchers reported on Friday.

They described the cases of 10 patients at a Michigan hospital who were so ill they had to be put on ventilators. Three died. Nine of the 10 were obese, seven were severely obese, including two of the three who died.

The study, published in advance in the Centers for Disease Control and Prevention’s weekly report on death and disease, also suggests doctors can safely double the usual dose of oseltamivir, Roche AG’s antiviral drug sold under the Tamiflu brand name.

“What this suggests is that there can be severe complications associated with this virus infection, especially in severely obese patients,” said CDC virus expert Dr. Tim Uyeki.

“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 said in a telephone interview.

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 with severe acute respiratory distress syndrome caused by infection with H1N1.

“Of the 10 patients, nine were obese (body mass index more than 30), including seven who were extremely obese (BMI more than 40),” they wrote in their report.

Their study was not designed to see if obesity or anything else poses a special risk factor for flu. But the researchers were surprised to see that seven of the 10 patients were extremely obese.

MULTIPLE ORGAN FAILURE

Nine had multiple organ failure, which can be seen in influenza, but five had blood clots in the lungs, and six had kidney failure.

None has fully recovered, the researchers said.

The H1N1 swine flu virus first emerged in Mexico in March and was spreading out of control in the United States by the time it was identified at the end of April. The World Health Organization declared a pandemic in June.

While it is causing moderate illness, all influenza viruses can be deadly and this one is no exception. It has killed close to 500 people globally, more than 200 in the United States alone.

However, the new virus has a slightly different pattern from seasonal flu -- it spreads in the summer months, attacks young adults and older children, and may affect the body slightly differently.

As with H5N1 avian influenza, which only rarely attacks people, patients seem to survive better if they get Tamiflu for longer than the usual five-day treatment course, Uyeki said.

“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.

“The high prevalence of obesity in this case series is striking,” the CDC’s commentary accompany the report reads.

“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.”