Mexican Flu Cases Show 1918-like Concentration of Severe Pneumonia in 20-44 Year Olds

July 1, 2009 (LPAC)—A study by U.S. and Mexican epidemiologists of the spring 2009 H1N1 outbreak in Mexico shows a remarkable shift in the ages of patients who become ill or die of severe pneumonia, especially concentrated in the 20-44 year old age bracket. While mortality rates from infection are, so far, well below the deadly 1918 pandemic, the pattern of age distribution is a warning sign experts are taking very seriously. In comparing these results of the earliest known outbreak of the new H1N1 flu to the 1918 pandemic, which took between 50 to 100 million lives, the authors say:

"Although the epidemiologic, clinical, and virologic characteristics of this outbreak are evolving, this wave of pneumonia is reminiscent of the initial phase of pandemics from the last century. These factors include the documentation of a novel pathogenic influenza virus and a shift in the ages of patients who become ill or die. Early waves of respiratory illness in young adults were observed before the 1918 influenza pandemic in New York, Copenhagen, the United Kingdom, and Geneva, as well as among members of the U.S. military. The apparent population effect of S-OIV [Swine Origin Influenza Virus] infection has several signature features of previous pandemics, including atypical timing and age shifts in disease severity."

Accompanying the report are charts showing that the percentage distribution of deaths from severe pneumonia is higher than that of the seasonal flu in all age groups from 5 to 59 years old. (With the seasonal flu, severe symptoms and death are most prevalent in very young children and the elderly.) Especially alarming was the high percentage distribution of severe illness and death in the 20-44 year old age brackets.

The study is reported in an article by Chowell, Bertozzi, et al. published online by the New England Journal of Medicine June 29, 2009.