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Severe pneumonia shifted to younger age groups during influenza A (H1N1) outbreak in Mexico
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During the 2009 influenza A (H1N1) outbreak in Mexico, higher mortality and morbidity from severe pneumonia were observed in individuals aged between 5 and 59 years, indicating a need for stronger prevention efforts.
From March 24 to April 29, researchers compared records of age-specific cases of pneumonia and influenza and their corresponding deaths with data from influenza seasons from 2005 to 2008. During the study period, 2,155 cases of severe pneumonia were reported to the National Epidemiological Surveillance System (SINAVE). Among these patients, 821 were hospitalized and 100 died.
Also, 8,817 nasopharyngeal specimens were submitted to the Epidemiological Reference Laboratory; 3,664 tested positive for influenza A. Among these specimens, 2,582 were confirmed as influenza A (H1N1), according to the researchers.
Data indicated that 87% of deaths from severe pneumonia were among patients aged between 5 and 59 years compared with 17% in this age group for the previous periods. Shifts in morbidity were also observed during the study period; 71% of severe pneumonia cases were found in individuals aged 5 to 59 years vs. 32% during the past periods.
Lower morbidity was observed among people aged 60 years and older. Childhood exposure to influenza A (H1N1) viruses among these people — who were born before the 1957 influenza H2N2 pandemic — may provide them with some form of protection, according to the researchers.
“If resources or vaccine supplies are limited, these findings suggest a rationale for focusing prevention efforts on younger populations,” they wrote.
Chowell G. N Engl J Med. 2009;361:674-679.
This article describes the dramatic increase in respiratory disease that accompanied the spread of novel A/H1N1 virus in Mexico during spring 2009. It also highlights the potential of new viral strains to spread rapidly, even during unusual seasons of the year, and the potential to tax medical resources with little advance warning. Seasonal influenza strains typically cause the highest attack rates in people aged younger than 5 years and the highest mortality rates in people aged older than 65 years. This report describes the most severe illness and the highest death rates due to the novel A/H1N1 virus in Mexico occurring in people aged between 20 and 45 years. This epidemiology is similar to the experience in the United States, which showed the highest attack rates in those aged between 5 and 24 years and the greatest number of deaths occurring in individuals aged between 25 and 49 years. In both countries, the attack and death rates due to novel A/H1N1 were lower in people aged older than 65 years than in most other age groups, suggesting some degree of pre-existing immunity among older individuals. This observation forms the basis for the recommendation by the CDC to prioritize monovalent novel A/H1N1 vaccine administration to people aged 6 months through 24 years; those aged 25 through 64 years, especially those with pre-existing medical conditions; pregnant women; people who live with or care for infants aged younger than 6 months and health care and emergency medical personnel. Because of pre-existing immunity, those aged older than 64 years will be in a lower priority group for vaccination with the monovalent novel A/H1N1 vaccine when it becomes available.
– H. Cody Meissner, MD
Professor of Pediatrics, Tufts University School of Medicine, Boston