Swine Flu Vaccination Should Target the Middle-Aged and Younger

The current novel swine-origin influenza A (H1N1) pandemic, the onset of which began in the spring of this year in Mexico, is similar to past pandemics in that the rate of severe pneumonia is higher among younger persons, compared with the age pattern consistent with the seasonal flu, according to an article published June 29 online ahead of print by the New England Journal of Medicine. These data suggest that persons who were exposed to H1N1 strains during childhood before the 1957 pandemic—the last time a genetically similar influenza strain predominated—have relative protection against severe disease, said researchers led by Gerardo Chowell, PhD, from the Mathematical, Computational, and Modeling Science Center at Arizona State University, School of Human Evolution and Social Change, Tempe.

“If resources or vaccine supplies are limited, these findings suggest a rationale for focusing prevention efforts on younger populations,” the investigators pointed out. Other institutions affiliated with this study were the NIH in Bethesda, Maryland, and the National Institute of Public Health in Mexico.

Dr. Chowell and colleagues analyzed data from Mexico’s National Epidemiological Surveillance System during the study period, defined as the date of the onset of symptoms in the patient with the earliest suspected case of atypical pneumonia (March 24) to April 29, when selective reporting of severe pneumonia ceased. Records showed that of 2,155 patients with severe pneumonia, 821 (38%) were hospitalized and 100 (5%) died. Only a portion of all hospitalization for pneumonia were captured, the researchers acknowledged; however, “it provided the best data currently available … during this period,” they added. For reference, the investigators also obtained data on pneumonia and influenza incidence and mortality in November and December, January through March, and November through February during the peak of the three preceding seasonal influenza seasons (2005 through 2008).

In the 5 to 59 age-group, the percentage of patients with severe pneumonia was significantly higher during the study period than on average during the referent periods (71% vs 32%). Similarly, 87% of deaths from severe pneumonia during the study period occurred in patients ages 5 to 59, compared with only 17% on average during the referent periods. In the subgroups of younger and older patients, the percentage of deaths decreased, compared to that during the referent period. Specifically, mortality rates during the study period were 5% in patients ages 4 and younger versus 17% for young children during the referent period. For patients ages 60 and older, the mortality rates ranged from 0% to 3% during the study period compared with 3% to 42% during the referent period.

“Our outline of the age-stratification profile of risk provides a possible foundation for control strategies on the basis of the biologic plausibility of partial protection form earlier exposure, the researchers suggested. “Further studies are under way in Mexico to elucidate other potential risk factors for severity of … [H1N1] infection to guide targeted control efforts.”

—Adriene Marshall

Suggested Reading
Chowell G, Bertozi S, Colchero MA, et al. Severe respiratory disease concurrent with the circulation of H1N1 influenza. N Engl J Med. 2009 Jun 29; [Epub ahead of print].

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