With the second wave of H1N1 infections having crested and now trending down in the U.S., leading epidemiologists are predicting that the pandemic could end up being ranked as the mildest in modern times. However, experts warn that the flu is notoriously unpredictable so don’t let your guard down…yet.  This study indicates that the death toll is likely to be far lower than the number of fatalities caused by past pandemics.   The predictions are being met with a mix of skepticism, relief and trepidation.

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Harvard researcher Marc Lipsitch, DPhil, and colleagues from the U.K. Medical Research Council and the CDC, released a study “The Severity of Pandemic H1N1 Influenza in the United States, from April to July 2009: A Bayesian Analysis” on PLOS Medicine detailing that the current statistics are revealing that it has been no more severe than an average flu season.

There are some pretty big caveats 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. “I think it is very likely to be the mildest pandemic on record,” said Marc Lipsitch. “This is a serious disease (however),” Lipsitch said, 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,” says Bell. “While most people who get this illness do OK, it can be very severe — and the severity is concentrated in younger people.”

Public health officials worry that people may become complacent about getting vaccinated, which could prove disastrous if a third wave of infections swells later this winter or the virus mutates into a more dangerous form. Lipsitch and others stressed that the multibillion-dollar vaccination campaign and other intense responses were appropriate, given the uncertainty of what the nation and world was facing.  “We got lucky,” Lipsitch said. “But if we didn’t have a plan in place and we had 60,000 or 70,000 deaths, people would have been justifiably outraged.”