A warning notice is seen at the entrance of the Universitaets-Klinikum hospital in Essen October 8, 2009. REUTERS/Ina Fassbender

The new H1N1 flu is strikingly different from seasonal influenza, killing much younger people than ordinary flu and often killing them very fast, World Health Organization officials said on Friday.

A review of studies done during the seven months the virus has been circulating shows it is usually mild, but can cause unusual and severe symptoms in an unlucky few, according to a WHO-sponsored meeting in Washington this week.

Participants who have managed such cases agreed that the clinical picture in severe cases is strikingly different from the disease pattern seen during epidemics of seasonal influenza, WHO's Dr. Nikki Shindo told the meeting.

Swine flu was declared a pandemic in June and has been circulating globally. WHO stopped trying to count cases, as there are nowhere near enough tests to formally diagnose everyone who gets sick.

Separately on Friday, Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention said 86 U.S. children had died of swine flu, most in the 5- to 17-year old age group that normally escapes serious bouts with flu.

In severe cases, patients generally begin to deteriorate around three to five days after symptom onset. Deterioration is rapid, with many patients progressing to respiratory failure within 24 hours, requiring immediate admission to an intensive care unit, Shindo said.

While most such patients need to be put on ventilators right away, some are not helped by this treatment, Shindo noted.

In some places, she said, emergency rooms have been overwhelmed with patients, many requiring critical care.

The good news is that quick treatment with the antiviral drugs oseltamivir, made by Roche AG under the brand name Tamiflu, and zanamivir, made by GlaxoSmithKline as Relenza, helps a great deal, she said.

Usually influenza is a disease of the upper respiratory tract, affecting the nose and throat. But H1N1 goes deeper, into the lungs.

This virus really likes the lower respiratory tract. That means this virus is very likely to cause viral pneumonia compared to seasonal influenza, Shindo told a news conference.

Among the doctors speaking to the WHO meeting was Dr. Anand Kumar of St. Boniface Hospital in Winnipeg, Manitoba, who reported swine flu's effects in the Journal of the American Medical Association this week.

At one point, 50 percent of the available ICU (intensive care unit) beds in the entire city were filled with H1N1 patients, Kumar said in a telephone interview. We basically maxed out our capacity.

Shindo said WHO was struggling to understand what the risk factors are for a serious bout with swine flu.

Although the exact role of obesity is poorly understood at present, obesity and especially morbid obesity have been present in a large portion of severe and fatal cases, Shindo said. Obesity has not been recognized as a risk factor in either past pandemics or seasonal influenza.