H1N1 Swine Flu Less Severe Than Feared

Swine Flu No More Deadly Than Seasonal Flu, but Victims Much Younger

Medically Reviewed by Louise Chang, MD on December 07, 2009
From the WebMD Archives

Dec. 7, 2009 – H1N1 swine flu won't be as severe as was feared, but the pandemic is nothing to sneeze at, new predictions suggest.

When the fall/winter wave of H1N1 swine flu is over, it will have been no more severe than an average flu season, predict Harvard researcher Marc Lipsitch, DPhil, and colleagues from the U.K. Medical Research Council and the CDC.

"The good news is that ... the severity of the H1N1 flu may be less than initially feared," Lipsitch says in a news release.

There are some big asterisks next to that prediction:

  • Most of the deaths and hospitalizations in a typical flu season are elderly people. Most of those killed or hospitalized in the H1N1 swine flu pandemic are children and young adults.
  • Deaths attributed to seasonal flu include heart attacks, strokes, and other fatal conditions triggered by the flu. Nearly all deaths attributed to H1N1 flu are due to flu or to bacterial complications of flu.
  • The new predictions would be four or five times higher in populations without access to mechanical ventilation or intensive care.
  • All bets are off if the H1N1 swine flu shifts to older populations.

Even so, the new numbers are cause for relief if not for celebration. Before the 2009 H1N1 swine flu came along, planners were preparing for a pandemic with a case/fatality ratio of 0.1% -- that is, for one death in every 1,000 symptomatic infections.

The Lipsitch team now calculates that the H1N1 swine flu has a case/fatality ratio no higher than 0.048% -- and maybe seven to nine times lower, depending on the methods used for calculation.

"This is a serious disease," Lipsitch says in the news release. He noted that between one in 70 and one in 600 people who fall ill with H1N1 swine flu will be hospitalized.

The CDC has been careful not to characterize the severity of the 2009 H1N1 pandemic. The new predictions are very much in line with CDC's working estimates, says Beth Bell, MD, MPH, associate director for science at the CDC's immunization and respiratory disease center.

"This study sends the message that this is primarily a young person's disease and highlights the importance of taking advantage of this window of opportunity to get the vaccine and take preventive measures," Bell tells WebMD. "While most people who get this illness do OK, it can be very severe -- and the severity is concentrated in younger people."

H1N1 Swine Flu: Same Lung Damage as 1918 Flu

Highlighting the H1N1 flu's ability to turn deadly is a new study from James R. Gill, MD, from the New York City Medical Examiner's office, and Jeffrey Taubenberger, MD, PhD, of the National Institutes of Health.

Detailed autopsies of 34 people who died of H1N1 swine flu show that the virus typically kills by damaging the upper airways, although damage in the lower airways and deep lung was not uncommon.

Strikingly, the damage was very familiar.

"This pattern of pathology in the airway tissues is similar to that reported in autopsy findings of victims of both the 1918 and 1957 influenza pandemics," Taubenberger says in a news release.

The Lipsitch study appears in the December issue of the online journal PLoS Medicine. The Gill study, released online today, will appear in the February issue of Archives of Pathology and Laboratory Medicine.

Show Sources


Presanis, A.M. PLoS Medicine, December 2009; vol 6.

Gill, J.R. Archives of Pathology and Laboratory Medicine, February 2010; vol 134, published online ahead of print.

News release, National Institutes of Health.

News release, Harvard School of Public Health.

Beth Bell, MD, MPH, associate director for Science, National Center for Immunization and Respiratory Diseases, CDC.

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