(Health.com)  -- An analysis of the sickest swine flu patients in Australia, Canada, Mexico, and New Zealand suggests that relatively healthy adolescents and young adults are among the most likely to get very sick after an H1N1 infection, a pattern similar to that seen in the 1918 influenza pandemic.



The most important message is that children should get the H1N1 vaccination, Dr. Neil Schachter said.



Almost all critically ill patients in the studies were sick for only a few days before rapidly progressing to more severe symptoms and respiratory failure, which required treatment with a breathing machine, according to three studies published in the Journal of the American Medical Association.



The mortality rate ranged from 14.3 percent to 41.4 percent, depending on the country. The findings may help shine some light on what the 2009 H1N1 flu season may bring, and who may be hit the hardest by the swine flu during the next few months.



"These studies are telling us that young people are at risk for bad complications of H1N1 and under usual circumstances, [seasonal] flu does not cause acute respiratory failure in younger people," says Dr. Neil Schachter, the medical director of the respiratory care department at Mount Sinai Medical Center, in New York City, and the author of The Good Doctor's Guide to Colds and Flu.



The analysis of cases in Australia and New Zealand looked at people who developed severe acute respiratory distress syndrome (a condition in which the lungs fill with fluid) and were put on a life-support system known as extracorporeal membrane oxygenation (ECMO). This artificial heart and lung machine system, which puts oxygen into the blood and then carries this blood to the body tissues, is considered risky and expensive; as a result, it is not readily available in every hospital. The mortality rate was 21 percent for these patients, although it may have been higher without this treatment, the authors say. Health.com: 8 ways swine flu is changing society



"These studies provide important signals about what clinicians and hospitals may confront in the coming months," Dr. Douglas B. White, and Dr. Derek C. Angus, of the University of Pittsburgh School of Medicine, write in an editorial accompanying the new studies. In young, healthy patients, H1N1 can quickly cause respiratory failure that can't necessarily be reversed with mechanical ventilation, although such patients are not currently a priority group for H1N1 vaccination, they note.



In the Canadian study, about 30 percent to 40 percent of the patients had lung disease, were obese, or had high blood pressure, a history of smoking, or diabetes. Overall, 14.3 percent of 168 critically ill people with confirmed or suspected H1N1 died within 28 days.



The mortality rate in the Mexican study was strikingly higher. In Mexico City, where the H1N1 pandemic was first reported, 41.4 percent of 58 critically ill people died within 60 days of developing the flu. Health.com: 10 best big cities for people with asthma



Those people who died from H1N1 got sicker earlier in the course of their illness, had extremely low levels of oxygen in their blood, and had multiple organ failure. Their average age was 44 years old, and 54 of 58 patients needed mechanical ventilation. Other signs of more severe H1N1 included fever and severe trouble breathing. Health.com: 10 ways you put yourself at risk for swine flu (without realizing it!)



In the Canadian study (in which the average age was 21.4), the critically ill tended to be hospitalized within four days of developing flu symptoms, and there was about a one-day lapse between hospital admission to intensive care unit (ICU) admission. As in the Mexico City study, younger patients with low blood oxygen and multisystem organ failure were hardest hit. What's more, the critically ill tended to require mechanical ventilation and rescue therapies to aid in breathing. Health.com: 8 causes of chronic cough



In both countries, the H1N1 outbreak lasted about three months.



"It is not clear of hospitals' need to invest in this ECMO technology because this has not proven very successful in other respiratory illnesses," says  Schachter. "We do know that
What is the mortality rate? how much does the mortality rate range?
14.3 percent to 41.4 percent, 14.3 percent to 41.4 percent,
question: What is the mortality rate?, answer: 14.3 percent to 41.4 percent, | question: how much does the mortality rate range?, answer: 14.3 percent to 41.4 percent,