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Severe Respiratory Disease in Mexico: Past vs. Present

During the spring 2009 outbreak of H1N1 influenza in Mexico, most deaths and cases of severe pneumonia occurred in younger individuals.

In Mexico, a dramatic increase in the number of individuals (mostly young persons) hospitalized for pneumonia during the spring of 2009 prompted immediate evaluation. Ultimately, a novel swine-origin influenza A (HIN1) virus (S-OIV) was identified and was found to be the underlying pathogen in most cases. Now, using data from the National Epidemiological Surveillance System and the Mexican Ministry of Health, researchers have compared age patterns of morbidity and mortality between cases of severe pneumonia reported from March 24 through April 29, 2009, (n=2155) and those reported from epidemics during the preceding 4 years.

The age distributions for morbidity and mortality during the 2009 outbreak were markedly different from those observed during the previous periods of epidemic influenza, with increased involvement of people aged 5 to 59. Seventy-one percent of the severe pneumonia cases in 2009 occurred in this age group, compared with an average of 32% during the previous periods. Similarly, patients aged 5 to 59 accounted for 87% of deaths from severe pneumonia in 2009, compared with an average of 13% during the previous periods.

Comment: In the 2009 outbreak of S-OIV infection in Mexico — the early phase of the current influenza pandemic — the rate of serious influenza-related complications, including death, was highest in individuals aged 5 to 59. Reminiscent of past pandemics, this pattern is in distinct contrast to what is seen with seasonal influenza epidemics, in which severe influenza complications disproportionately affect older people. Epidemiologic findings, such as those presented here, should be key in devising vaccination strategies.

— Larry M. Baddour, MD

Published in Journal Watch Infectious Diseases July 1, 2009

Citation(s):

Chowell G et al. Severe respiratory disease concurrent with the circulation of H1N1 influenza. N Engl J Med 2009 Jun 29; [e-pub ahead of print]. (http://dx.doi.org/10.1056/NEJMoa0904023)