Despite the considerable amount of concern generated by the H1N1 outbreak earlier this year, the authors of a new study featured in PLoS Medicine believe it's impact will be relatively mild this flu season, with the number of hospitalizations and deaths related to the virus likely to remain much lower than anticipated.

"As more detailed data have become available, we have been able to improve our estimates of how severe this disease is. Early on, it was difficult to measure the flu's impact and it was crucial to plan for the full range of possible outcomes. 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 (1) hospitalized, (2) in an intensive-care unit (ICU) or on a ventilator or (3) had died. They analyzed data from the 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 July 2009. 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% of patients with symptoms of H1N1 flu during the April-July time period were hospitalized, 0.239% required intensive care or mechanical ventilation and 0.048% 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.

"The good news is that, along with previous work by the CDC and others, our work shows that the severity of the H1N1 flu may be less than initially feared," said Lipsitch. However, he adds that between 1 in 70 and 1 in 600 people who are sick with the illness will be hospitalized, and a fraction of those will die.

While the news is better than was thought at the start of the pandemic, the researchers emphasizes that it remains important to continue to vaccinate against H1N1 and to remain vigilant about disease in groups at risk of complications. "The U.S. Centers for Disease Control (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," said Lipsitch

Citation: Anne M. Presanis, Daniela De Angelis, Angela Hagy, Carrie Reed, Steven Riley, Ben S. Cooper, Lyn Finelli, Paul Biedrzycki, Marc Lipsitch, 'The Severity of Pandemic H1N1 Influenza in the United States, from April to July 2009: A Bayesian Analysis', PLos Medicine, 2009, doi: 10.1371/journal.pmed.1000207