* Most patients were under age 59
* All hospitalized patients had pneumonia
* No single pattern predicts sickest patients
By Maggie Fox, Health and Science Editor
WASHINGTON, June 29 (Reuters) - Swine flu patients in Mexico were young and many were healthy before developing severe infections, doctors reported on Monday.
The first detailed studies of the outbreak of a new strain of H1N1 influenza show the epidemic in Mexico resembled the early stages of other pandemics, and showed there is no way yet to predict who will become severely ill from the virus.
The World Health Organization has confirmed 70,893 cases in the new H1N1 swine flu pandemic, with 311 deaths. However, U.S. health officials last week said there were likely at least a million cases there alone. Iraq, Lithuania, Monaco and Nepal all confirmed their first cases on Monday.
Dr. Rogelio Perez-Padilla of the National Institute of Respiratory Diseases in Mexico City and colleagues studied 18 H1N1 cases in March and April, more than half of them aged 13 to 47.
Only eight had pre-existing medical conditions that might worsen their flu infection, they wrote -- including high blood pressure, diabetes, asthma and sleep apnea. Seven died -- all of multiple organ failure.
The doctors said 90 percent of the seriously ill patients were under 50 -- in contrast to seasonal influenza, which causes mostly mild illness in people under the age of 65.
"Most of our patients were young to middle-aged and had previously been healthy," they wrote in their report, published in the New England Journal of Medicine.
"One contributing factor for death in our patients may have been delayed admission and delayed initiation of oseltamivir."
Oseltamivir, sold by Roche AG ROG.VX under the brand name Tamiflu, can treat influenza, although Denmark reported the first case on Monday of swine flu resisting the drug’s effects. [ID:nLT265113]
"We did not find a factor that, before the onset of illness, predicted a worse outcome or death among our patients," Perez-Padilla’s team wrote.
In addition, 22 of 190 healthcare workers who came close to the patients themselves got flu-like illness but were treated with Tamiflu and none got seriously ill.
Dr. Stefano Bertozzi of the National Institute of Public Health in Mexico and colleagues studied the cases of 2,155 people who developed severe pneumonia from H1N1 infection in March and April, 821 who had detailed hospital records and 100 who died.
They found that 87 percent of those who died were aged 5 to 59, and 71 percent of severe cases were among people 5 to 59, compared to a usual average of 32 percent for seasonal flu.
"This wave of pneumonia is reminiscent of the initial phase of pandemics from the last century," they wrote.
Health experts have speculated that people over the age of 52 have some protection from the new virus because it may resemble a strain of H1N1 flu that circulated before 1957.
"Influenza A H1N1 abruptly disappeared from humans in 1957 and was replaced by a new reassortant virus that combined genes from the H1N1 strain and an avian virus," Dr. Shanta Zimmer and Dr. Donald Burke of the University of Pittsburgh wrote in a second report in the same journal. Flu viruses frequently swap genes in a process called reassortment.
(Editing by Vicki Allen)