These studies used mice and ferrets, which are known to be useful in predicting how flu viruses can affect humans. The researchers also tested the viruses in non-human primates.

To date, the H1N1 (pandemic) flu has not been any worse than seasonal flu.

That could be about to change.

If what researchers saw in these studies is an accurate predictor of what to expect in the coming winter months, patients will be sicker.

- The second round in pandemics has historically hit patients harder than the first. That pattern appears to be more likely based on these studies although more studies are needed.

- As noted in the Comment, cold dry air is a virus’s favorite climate. Winter months are ahead.

Physician’s First Watch

“A Japanese team has examined whether pandemic influenza A (H1N1) virus is more virulent than a widely circulating seasonal H1N1 human flu virus. The studies were conducted in two animal models (mice and ferrets) that traditionally have been good predictors of human infection with flu viruses; the viruses also were tested in nonhuman primates.” (Emphasis added)

…”Comment: Flu caused by pandemic influenza A (H1N1) virus has not been more severe than that caused by seasonal flu viruses. Yet, this study suggests that the pandemic H1N1 virus is more pathogenic. Should we be reassured by the benign illness caused by this virus since it began circulating in March 2009 or be worried by these animal data? I think we should be worried. Flu viruses are more transmissible and virulent in cold dry air. This virus emerged and spread in the northern hemisphere in late spring and summer — it has not had an opportunity to show its worst.” (Emphasis added)

— Anthony L. Komaroff, MD

Source: Journal Watch General Medicine, September 10, 2009

Citation: Nature 2009 Aug 20; 460:1021.