MONDAY, June 29 (HealthDay News) -- With a worldwide pandemic under way and more than a million Americans sickened by the new swine flu, the special nature of this disease is becoming better understood.

Several articles published online Monday by the New England Journal of Medicine show that, unlike seasonal flu, the new H1N1 flu strain attacks younger people and can be more severe and deadly in that group. The reports suggest a possible vaccination policy and also account for some reasons that this strain of flu appears milder than that of other pandemics.

"These findings are in keeping with the fact that new strains or pandemic strains tend to be more deadly in younger patients," said flu expert Dr. Marc Siegel, an associate professor of medicine at New York University's Langone Medical Center in New York City.

Also Monday, health officials in Denmark reported what is believed to be the first case of someone with a strain of swine flu that's resistant to Tamiflu, an antiviral medication.

Though the H1N1 flu has been mild for most people, some have developed pneumonia and respiratory distress, which can be severe and even fatal. Most such cases have been confined to young and middle-aged people, many of them otherwise healthy.

One report targeted the initial flu outbreak in Mexico, which included 2,155 cases of swine flu reported by the end of April. Researchers focused on the 100 people who died and what caused those deaths.

They found that 87 percent of the deaths and 71 percent of the cases of pneumonia were seen in people aged 5 to 59 years. That's unlike what is seen with seasonal flu epidemics, in which, on average, 17 percent of those in that age range who are seriously ill die and 32 percent develop severe pneumonia.

The findings are similar to other flu pandemics, which have affected mostly younger people, the researchers said. Older people have some protection from the H1N1 strain because they have been exposed to earlier strains of H1N1 flu in childhood, specifically before the 1957 flu pandemic.

Given this, the researchers say, younger people should be vaccinated first when a vaccine becomes available, particularly if it is in limited supply, because they are most likely to get and spread the disease.

In another report in the journal, Mexican researchers also looked at the age distribution of those who died or developed acute respiratory distress from the new H1N1 flu. Of the 18 people with pneumonia hospitalized in April with the flu at the National Institute of Respiratory Diseases in Mexico City, more than half were 13 to 47 years old and only eight had a preexisting condition, the researchers found.

"The main finding is the capability of H1N1 of producing severe damage to previously healthy individuals," said the study's lead researcher, Dr. Rogelio Perez-Padilla, from the Mexico City institute. "Of course, some of the patients who died had chronic diseases, and they are in a higher risk, but the virus may affect healthy people," he said.

"Do not disregard the epidemic as mild or irrelevant," Perez-Padilla urged. "This has not been the case for an increasing number of individuals with severe disease and may change with time. Even for patients with severe disease there is hope, but unfortunately, we have to expect deaths in previously healthy individuals."

Siegel thinks that everyone should be vaccinated against this flu. "We still have to protect people with chronic illnesses, pregnant women, the very young and the very old," he said.

"The best way to protect any population is with herd immunity," Siegel said. "The goal of getting the vaccine is not to protect you, it's to protect you by getting everybody the vaccine, which decreases circulating virus."

Siegel predicted that a large outbreak of the H1N1 swine flu would occur in the fall. "But it is probably the mildest pandemic virus in recent history, and that's a good thing," he said. "I don't expect it to be morphing into a massive killer, but I expect it will come back and spread."

A vaccine, however, could stem the tide of the virus, he said.

"This is one of the first times in history that we will be able to target an emerging, although mild, pandemic strain and, by vaccinating the population against it, severely limit its spread," he said.

Another article in the journal, from researchers at the U.S. National Institutes of Health, compares the new H1N1 flu to the deadly 1918 flu, which killed between 20 million and 40 million people worldwide, including 500,000 in the United States.

"The 1918-1919 influenza pandemic was a defining event in the history of public health," co-author Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, said in a prepared statement. "The legacy of that pandemic lives on in many ways, including the fact that the descendants of the 1918 virus have continued to circulate for nine decades."

The authors say that descendants of the 1918 flu, which include the new H1N1 strain, have genetically modified themselves to be better able to survive and spread. To do this, they have become less severe so as not to kill their host, making it easier to spread from person to person.

A final report in the journal is from researchers at the U.S. Naval Health Research Center in San Diego and the U.S. Centers for Disease Control and Prevention, who developed a rapid test to diagnose the new H1N1 swine flu.

They note that although the test was effective in finding the first cases of the strain in the United States, their test and others like it must be constantly updated to keep up with changes in this and other influenza strains.

In the case of resistance to Tamiflu reported Monday by Danish officials, the person reportedly was taking the drug because of believed exposure to swine flu but developed the disease anyway.

A representative of Roche, which makes Tamiflu, said in a teleconference with journalists that such a development was not unexpected.

Officials said the person who'd contracted swine flu had recovered.

More information

The U.S. Centers for Disease Control and Prevention have more on swine flu.

SOURCES: Marc Siegel, M.D., associate professor of medicine, New York University Langone Medical Center, New York City; Rogelio Perez-Padilla, M.D., National Institute of Respiratory Diseases, Mexico City; June 29, 2009, New England Journal of Medicine, online

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