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Early Estimate of Pandemic Potential of Influenza A H1N1 "Swine Flu"

The current virus is transmitted efficiently but probably is less lethal than past pandemic viruses.

A team of epidemiologists has analyzed the influenza A (H1N1) epidemic in Mexico. Data related to the outbreak were collected primarily in April and early May 2009. The researchers presented several tentative conclusions:

• Cases outside of Mexico occurred most commonly in countries that had the highest volume of travelers from Mexico.
  
• Attack rates of clinical disease are higher in children younger than 15 years than in adults (relative risk for children, 1.52). This finding suggests that, although the virus is novel, adults might have some protection due to cross-immunity from exposure to strains that have circulated in the past.
  
• The virus is transmitted more efficiently from person to person than are usual seasonal flu viruses.
  
• The estimated fatality rate is 0.4% — this virus is considerably less lethal than the virus that caused the 1918–1919 pandemic but somewhat more lethal than usual seasonal flu viruses.
  

Comment: These data indicate that the current H1N1 virus is more transmissible and possibly more lethal than regular seasonal flu viruses, but it is considerably less transmissible and lethal than the catastrophic 1918–1919 pandemic virus. However, influenza viruses mutate rapidly, and this virus could change considerably in the coming months. The 1918–1919 virus also was mild when it first appeared in the spring of 1918, but it killed about 3% of people that it infected when it returned the next winter.

— Anthony L. Komaroff, MD

Published in Journal Watch General Medicine May 21, 2009

Citation(s):

Fraser C et al. Pandemic potential of a strain of influenza A (H1N1): Early findings. Science 2009 May 14; [e-pub ahead of print]. (http://dx.doi.org/10.1126/science.1176062)

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