WASHINGTON, Dec. 8 (Xinhua) -- A new study from researchers at the Harvard School of Public Health (HSPH) and the UK Medical Research Council projects that the severity of the H1N1 flu during the autumn-winter flu season in the United States will likely be less than previously feared.

    The estimates of hospitalizations and life-threatening events in the study are the most accurate to date of the H1N1 pandemic's impact in the United States. The study appears online on Monday in the journal PLoS Medicine.

    "As more detailed data have become available, we have been ableto improve our estimates of how severe this disease is. Fortunately, the virus now appears to be near the milder end," said Marc Lipsitch, professor of epidemiology at HSPH and the study's senior author.

    The researchers measured the severity of the pandemic by analyzing what fraction of people in the study population that were sick with H1N1 flu were hospitalized; in an intensive-care unit or on a ventilator; or had died.

    They analyzed data from the U.S. Centers for Disease Control and Prevention (CDC) and from New York and Milwaukee, two cities where health officials collected particularly high-quality surveillance data during the wave of infections from April to July2009.

    The researchers then combined all the evidence using a statistical approach called Bayesian evidence synthesis to estimate the probability of the three outcomes above for individuals who fell ill with H1N1 in the overall U.S. population and by age group.

    The researchers used two different approaches to estimate these risks. One approach led to an estimate that approximately 1.44 percent of patients with symptoms of H1N1 flu during the April-July time period were hospitalized, 0.239 percent required intensive care or mechanical ventilation and 0.048 percent died. The other estimate found probabilities 7 to 9 times lower, due to the use of different data to estimate how many individuals were sick.

    Based on these findings and assuming that the virus doesn't change its characteristics, the researchers estimate that the severity of the autumn-winter pandemic wave of H1N1 flu could have a death toll in a range from considerably below the estimated 36,000 associated with an average flu season in the U.S. to slightly higher.

    Also, unlike seasonal flu, which kills mainly elderly adults, the H1N1 flu could have the greatest impact in children aged 0-4 and especially adults 18-64, a shift toward nonelderly persons that has been seen in prior flu pandemics.

    While the news is better than was thought at the start of the pandemic, Lipsitch emphasizes that it remains important to continue to vaccinate against pandemic H1N1 and to remain vigilant about disease in groups at risk of complications.

    "This is a serious disease," said Lipsitch. "The CDC and others have shown that certain high-risk groups, including pregnant women, people with asthma, and people with compromised immune systems, should be vaccinated and should seek prompt treatment if they suspect they are sick with H1N1. Even for people outside these high-risk groups, vaccination is an important way to reduce the risk of what can be a serious illness."