WASHINGTON (Reuters) – 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.”

(With additional reporting by Julie Steenhuysen in Chicago; Editing by Doina Chiacu)

WASHINGTON (Reuters) – Most of the people who have died from the new pandemic H1N1 flu had underlying conditions such as asthma, but 45 percent seemed healthy, according to the largest study yet of U.S. cases.

Children with sickle cell and other blood diseases have a special risk from the swine flu, just as they do from seasonal influenza, Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention said on Tuesday.

She said injectable versions of the flu vaccine — suitable for babies, people with asthma and people 50 and older — will be available this week.

Schuchat said the CDC collected detailed data on 1,400 adults and 500 children hospitalized with swine flu in 10 states. The findings confirm that most serious cases and deaths have been in people under the age of 65.

“The vast majority of hospitalizations and deaths are occurring in younger people,” Schuchat told reporters in a telephone briefing. Five more children have died, bringing the H1N1 death toll among children in the United States to 81.

She said 55 percent of the adults had a condition known to worsen flu of all kinds. “In adults, the most common underlying conditions were asthma and chronic lung disease, chronic heart disease and immunosuppression,” Schuchat said.

Six percent were pregnant. Pregnant women have suppressed immune systems so their bodies do not reject the baby, and may also have pressure on the lungs from the fetus.

“And in children, the most common underlying conditions were asthma and chronic lung disease, neurological or neuromuscular diseases, and sickle cell or other blood disorders.”

Schuchat said 5.8 percent of hospitalized children had a blood disease related to red blood cells, such as sickle cell disease.

The CDC had not mentioned sickle cell disease before as a special risk, but such children had been highlighted in influenza guidelines as being at special risk and needing to be vaccinated every year.

9.8 MILLION DOSES

Schuchat said the vaccination program was continuing slowly. The CDC has opted to start immunizing people as soon as vaccine becomes available, which means supply has been spotty.

“As of yesterday, 9.8 million doses of the H1N1 vaccine were available to be ordered,” Schuchat said. States have ordered 5.8 million of these doses to go to providers.

“I’m happy to say that about half of the vaccine that’s available for order is now the injectable form,” she said.

So far the vaccine available for swine flu has been only AstraZeneca unit MedImmune’s nasal spray vaccine, which is only approved for people aged 2 to 49 without asthma or other lung conditions. States have been giving it to healthcare workers or older children.

Schuchat agreed that some places had experienced trouble getting either H1N1 or seasonal influenza vaccine quickly.

“It does take time to process the orders, to package them for the right amounts that are heading out toward the many sites we’ll be delivering vaccine, and this is going to be ongoing over the next days and weeks,” she said.

Vaccine would be more widely available at the end of October, Schuchat said.

She said there was no rush for people to be vaccinated against seasonal influenza.

“Right now we’re seeing the H1N1 strains. We aren’t seeing much at all of the seasonal strains. And we think there’s time to be vaccinated against the seasonal flu. Even if more vaccine isn’t available until November or December, we think it will be just fine to be vaccinated then,” she said.

(Editing by Mohammad Zargham)