WASHINGTON (Reuters) - Autopsies on people who have died from the new pandemic H1N1 flu show this virus is different from seasonal influenza, even if it has not yet caused more deaths, experts told a meeting on Tuesday.

Americans who died from swine flu had infections deep in their lungs, Dr. Sherif Zaki of the U.S. Centers for Disease Control and Prevention told a meeting of flu experts, including damage to the alveoli -- the structures in the lung that deliver oxygen to the blood.

This in turn caused what is known as acute respiratory distress syndrome -- an often fatal development that leaves patients gasping for breath.

The World Health Organization has confirmed 3,205 deaths globally from swine flu but experts agree all estimates of the extent of the pandemic are grossly understated because so few patients are ever actually tested.

Seasonal flu kills, too -- about 250,000 to 500,000 cases a year globally, according to the WHO. But not in the same way as swine flu, which unlike seasonal flu frequently causes severe disease in young adults and children.

"It is very rarely you see what we call diffuse alveolar damage in fatal seasonal influenza," Zaki told a meeting sponsored by the U.S. Institute of Medicine, which advises government on health matters.

Seasonal flu causes bronchitis and other upper respiratory disease. But Zaki, the chief infectious disease pathologist at CDC, said the new virus had burrowed into the lungs of the 90 or so people he examined after they died, and they had huge amounts of the virus in their blood.

"This is almost exactly what we see with avian flu," Zaki said. "This looks like avian flu on steroids."

EXPERIMENTAL DRUGS

Dr. Yoshi Kawaoka of the University of Wisconsin said tests in monkeys showed the virus lives and replicates 1,000-fold better in the lungs than does seasonal flu.

He said the No. 1 drug of choice against H1N1 -- Roche AG's and Gilead Sciences Inc's Tamiflu -- lowered the so-called viral load of virus in the lungs just enough to help the body fight back.

Experimental flu drugs lower it even more, notably Daiichi Sankyo Co Ltd's CS 8958 and another drug called T-705 or favipiravir, made by Fujifilm Holdings Corp unit Toyama Chemical Co, Kawaoka said.

Zaki said 90 percent of the fatalities he looked at had some condition that would predispose them to serious disease. They had a median age of 38 and one victim was a two-month-old infant who died within a day of getting sick.

Nearly half -- 46 percent -- were obese, many had fatty liver disease, 27 percent had heart disease and 22 percent had asthma, he said.

Dr. Guillermo Ruiz-Palacios of Mexico's National Institute of Medical Sciences and Nutrition said many Mexican patients with severe disease were also obese. In addition, patients came in late for treatment and many were infected with a second common virus, called parainfluenza virus. 

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Technician Heather Bickley tested samples for H1N1 flu at the Arizona State Health Lab in Phoenix. At least 26 states are now reporting widespread flu activity, federal officials said. (Matt York/ Associated Press)

In Austin, so many parents are rushing their children to Dell Children’s Medical Center of Central Texas with swine flu symptoms that the hospital had to set up tents in the parking lot to cope with the onslaught.

In Memphis, the Le Bonheur Children’s Medical Center emergency room got so crowded with feverish, miserable youngsters that it had to do the same thing.

And in Manning, S.C., a private school where an 11-year-old girl died shut down after the number of pupils who were out sick with similar symptoms reached nearly a third of the student body.

“It just kind of snowballed,’’ said Kim Jordan, a teacher at Laurence Manning Academy, which closed Wednesday after Ashlie Pipkin died and the number of ill students hit 287. “We had several teachers out also.’’

“H1N1 is spreading widely throughout the US,’’ said Thomas Frieden, director of the federal Centers for Disease Control and Prevention in Atlanta. The CDC reported last week that at least 26 states are now reporting widespread flu activity, up from 21 a week earlier.

Even though some parts of the Southeast that started seeing a surge of cases first now seem to be showing a decline, that could be a temporary reprieve, Frieden said. And other parts of the country are probably just starting to feel the second wave.

“Influenza is perhaps the most unpredictable of all infectious diseases,’’ Frieden said.

The pandemic has prompted scattered school closings around the country in recent weeks, including 42 schools that closed in eight states Friday, affecting more than 16,000 students.

Many colleges and universities have been hit particularly hard, forcing some to open separate dorms for sick students. Ninety-one percent of the 267 colleges and universities being surveyed by the American College Health Association are now reporting cases.

The government is starting an unprecedented system to track possible side effects as mass flu vaccinations begin next month. The idea is to detect any rare but real problems quickly, and explain the inevitable coincidences that are sure to cause some false alarms.

In just a few months, health authorities hope to vaccinate well over half the population against swine flu, which doctors call the 2009 H1N1 strain. No more than 100 million Americans usually get vaccinated against regular winter flu, and never in such a short period.

The last mass inoculations against a different swine flu, in 1976, were marred by reports of a rare paralyzing condition, Guillain-Barré syndrome, a sometimes fatal paralysis. Other possible side effects could potentially include heart attacks, strokes, seizures, and miscarriages.

On top of routine vaccine tracking, these government-sponsored monitoring projects are planned:

■ Harvard Medical School scientists are linking large insurance databases that cover up to 50 million people with vaccination registries around the country for real-time checks of whether people see a doctor in the weeks after a flu shot and why. The huge numbers make it possible to quickly compare rates of complaints among the vaccinated and unvaccinated, said the project leader, Dr. Richard Platt, Harvard’s population medicine chief.

■ Johns Hopkins University will direct e-mails to at least 100,000 vaccine recipients to track how they’re feeling, including the smaller complaints that wouldn’t prompt a doctor visit. If anything seems connected, researchers can call to follow up with detailed questions.

■ The Centers for Disease Control and Prevention is preparing take-home cards that tell vaccine recipients how to report any suspected side effects to the nation’s Vaccine Adverse Event Reporting system.

“Every day, bad things happen to people. When you vaccinate a lot of people in a short period of time, some of those things are going to happen to some people by chance alone,’’ said Dr. Daniel Salmon, a vaccine safety specialist at the Department of Health and Human Services.

Material from the Associated Press was included in this report.

Swine flu put many hospitalized patients into ICU

One quarter of Americans sick enough to be hospitalized with swine flu last spring wound up needing intensive care and 7 percent of them died, the first such study of the early months of the global epidemic suggests.

That's a little higher than with ordinary seasonal flu, several experts said.

What is striking and unusual is that children and teens accounted for nearly half of the hospitalized cases, including many who were previously healthy. The study did not give a breakdown of deaths by age.

"Contrary to the perception among many people that this influenza, novel H1N1, is mild, these data vividly demonstrate that influenza can make you very, very ill," said Dr. William Schaffner, a Vanderbilt University flu expert and spokesman for the Infectious Diseases Society of America.

"Clearly, the best way to protect yourself and your family is to get vaccine as soon as it becomes available," said Schaffner, who had no role in the study but has consulted for swine flu vaccine makers.

The study was done by researchers at the U.S. Centers for Disease Control and Prevention, working with hospitals and state and local health departments. Results were published online Thursday by the New England Journal of Medicine.

A second study released by the journal revealed that swine flu had a profound effect on intensive care units in Australia and New Zealand from June through August - winter months and the normal flu season in the Southern Hemisphere.

"They had a 15-fold increase in ICU care. That's a 1,500 percent increase," said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. "We're still heading into our flu season. Will we see this same big increase in cases over the next several months or will we have a peak in cases in October or November?"

In the U.S. study, researchers led by the CDC's Dr. Seema Jain identified 272 patients hospitalized for at least a day from April through mid-June, when the novel virus caused its first wave of cases. That's about one-fourth of the total hospitalizations for swine flu reported during that time, but researchers only studied lab-confirmed cases and patients who agreed to be part of the study.

Three-fourths of these patients had other health problems, such as diabetes - typical of seasonal flu, too. However, only 5 percent were 65 and older; ordinary seasonal flu usually hits hardest in the elderly.

Seven percent were pregnant, even though pregnant women make up only 1 percent of the general population. Among hospitalized patients whose weight was known, 26 percent were very obese, confirming a risk factor suggested by other information from the outbreak.

Symptoms were different - 39 percent had diarrhea or vomiting versus only 5 percent with regular flu. About 40 percent had pneumonia. Seven percent died, and all had been put on breathing machines.

About three-fourths of hospitalized patients were given Tamiflu or other antiviral medicines, although most did not get these within the two days of first symptoms, as doctors recommend. Survival appeared to be better among those who got treatment quickly.

"The use of antivirals is critical," Jain said. "Start them, start them early. The patients who are hospitalized should get them quickly."

Other experts caution against making too much of specific numbers from these early results.

"We don't know how good these numbers are. They've done a good job; it's the best that we've got," Osterholm said. "But there are deaths out there that are not being recognized as influenza - only an autopsy would pick them up. And there are likely hospitalizations for flu that were missed as flu."

The Southern Hemisphere study involved 722 patients with confirmed cases of swine flu who were treated in intensive care units. More than 14 percent, or one in seven, died. That study also found a disproportionately large number of cases in pregnant women and the very obese.

In children, other, much rarer chronic conditions, such as sickle cell anemia, cerebral palsy and muscular dystrophy, are also predisposing patients to life-threatening bouts of the virus, federal health officials said.

Epidemiologists at the Centers for Disease Control and Prevention studied the experience of about 1,400 people older than 18 and 500 children who had been hospitalized in 10 states since the new influenza strain emerged in April.

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