jueves, 13 de agosto de 2009

Severe Respiratory Disease Concurrent with the Circulation of H1N1 Influenza

ABSTRACT

Background In the spring of 2009, an outbreak of severe pneumonia was reported in conjunction with the concurrent isolation of a novel swine-origin influenza A (H1N1) virus (S-OIV), widely known as swine flu, in Mexico. Influenza A (H1N1) subtype viruses have rarely predominated since the 1957 pandemic. The analysis of epidemic pneumonia in the absence of routine diagnostic tests can provide information about risk factors for severe disease from this virus and prospects for its control.

Methods From March 24 to April 29, 2009, a total of 2155 cases of severe pneumonia, involving 821 hospitalizations and 100 deaths, were reported to the Mexican Ministry of Health. During this period, of the 8817 nasopharyngeal specimens that were submitted to the National Epidemiological Reference Laboratory, 2582 were positive for S-OIV. We compared the age distribution of patients who were reported to have severe pneumonia with that during recent influenza epidemics to document an age shift in rates of death and illness.

Results During the study period, 87% of deaths and 71% of cases of severe pneumonia involved patients between the ages of 5 and 59 years, as compared with average rates of 17% and 32%, respectively, in that age group during the referent periods. Features of this epidemic were similar to those of past influenza pandemics in that circulation of the new influenza virus was associated with an off-season wave of disease affecting a younger population.

Conclusions During the early phase of this influenza pandemic, there was a sudden increase in the rate of severe pneumonia and a shift in the age distribution of patients with such illness, which was reminiscent of past pandemics and suggested relative protection for persons who were exposed to H1N1 strains during childhood before the 1957 pandemic. If resources or vaccine supplies are limited, these findings suggest a rationale for focusing prevention efforts on younger populations.


In early April 2009, a sharp increase in reports of patients requiring hospitalization for pneumonia and an unusual series of deaths were reported to the Mexican Ministry of Health. The National Epidemiological Surveillance System (SINAVE), a nationwide interagency system led by the Directorate General of Epidemiology,1 noted a particular increase among adults between the ages of 20 and 40 years and an increase in cases of laboratory-confirmed influenza. Typically, in Mexico, seasonal influenza is observed from October through March,2 with an appreciable increase in the rate of death among the elderly,3 similar to the pattern observed in other temperate climates, such as the United States.4,5 The concurrent finding of a swine-origin influenza A (H1N1) virus (S-OIV)6 from infected children in the United States7 prompted a rapid response from the Mexican public health emergency system.

From March 24 through April 29, 2009, there were reports of 2155 cases of severe pneumonia, including 100 deaths, to the SINAVE system in response to requests for data on patients who had required hospitalization for severe pneumonia. During this period, of the 8817 nasopharyngeal specimens that were submitted to the National Epidemiological Reference Laboratory, 3664 (42%) tested positive for influenza subtype A; of these specimens, 2582 (70%) were confirmed as S-OIV by reverse-transcriptase–polymerase-chain-reaction (PCR) assay.

In this article, we evaluate the reported case series of severe pneumonia and compare the age patterns with respect to morbidity and mortality with patterns from recent influenza epidemics in Mexico. Crucial epidemiologic factors, such as the age pattern of morbidity and mortality during the 2009 epidemic, are inferred because of the limited availability of diagnostic tests and data. Given the relatively rapid global spread of this newly described pathogen,8 early identification of groups at risk for severe pneumonia can aid in prioritizing the use of vaccines and antiviral drugs in the face of limited supplies.

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