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The CDC’s Report on the New Swine-Origin Influenza A (H1N1) Virus

The formal surveillance report of the widely reported cases of S-OIV infection

The CDC has published the data and epidemiology for confirmed cases of human infection with the new circulating swine-origin influenza A (H1N1) virus (S-OIV) in the U.S. The virus contains genes from swine, avian, and human influenza. This strain was detected when routine public health surveillance indicated that a collected isolate tested positive for influenza A virus but negative for human H1 or H3 subtypes. Modifications of existing assays were developed to detect S-OIV by reverse-transcriptase–polymerase chain reaction (RT-PCR).

A total of 642 cases of S-OIV infection from 41 states were confirmed from April 15 through May 5, 2009; 104 cases were from four clusters in schools or universities. The age range of patients was 3 months to 81 years; 40% were between ages 10 and 18 years, and only 5% were older than 50. Of the 381 patients with complete data, 68 (18%) had recently traveled to Mexico. Predominant symptoms were fever (94%), cough (92%), and sore throat (66%). Vomiting and diarrhea were less common (25%). Thirty-six patients (9%; age range, 19 months to 49 years) were hospitalized. Among 22 hospitalized patients with complete data, 9 (41%) had a standard risk factor for severe seasonal influenza, 11 developed pneumonia, and 8 required admission to an intensive care unit. As of May 5, 2009, 18 hospitalized patients (82%) had recovered, 2 remain critically ill, and 2 had died (1 infant with myasthenia gravis and 1 pregnant woman).

All 642 patient isolates were PCR positive for S-OIV. All 49 isolates that were sequenced in all eight segments of influenza virus were 99% to 100% identical to each other and were distinct from isolates obtained from the rare cases of swine flu detected in the CDC surveillance system since 2005 (JW Infect Dis May 7 2009). All isolates are sensitive to neuraminidase inhibitors.

Comment: This paper is the formal surveillance report of early cases of S-OIV. The identification of a new virus subtype with documented transmission from person to person and the young age of patients are features that are similar to those of previous "pandemics." The noted clinical manifestations appear typical of seasonal influenza, although data on the prevalence of other common symptoms, including headache and myalagias, are not yet available. Whether other features of this outbreak, such as successive waves of illness and increased mortality, will develop is uncertain. Two accompanying perspectives and an editorial explore how the Internet can help track data more quickly, how such information might affect policy planning, and what types of additional studies and information are needed. This experience provides a "teachable moment" to remind patients about the risk for routine seasonal influenza (which causes, on average, 36,000 deaths/year) and the importance of vaccination, hand hygiene, and proper respiratory etiquette for those with upper respiratory infections. The New England Journal of Medicine and Journal Watch have created an H1N1 influenza center to monitor new developments.

— Peggy Sue Weintrub, MD

Published in Journal Watch Pediatrics and Adolescent Medicine May 8, 2009

Citation(s):

Dawood FS et al. Emergence of a novel swine-origin influenza A (H1N1) virus in humans. N Engl J Med 2009 May 7; [e-pub ahead of print]. (http://dx.doi.org/10.1056/NEJMoa0903810)

Belshe RB. Implications of the emergence of a novel H1 influenza virus. N Engl J Med 2009 May 7; [e-pub ahead of print]. (http://dx.doi.org/10.1056/NEJMe0903995)

Miller MA et al. The signature features of influenza pandemics — Implications for policy. N Engl J Med 2009 May 7; [e-pub ahead of print]. (http://dx.doi.org/10.1056/NEJMp0903906)

Brownstein JS et al. Digital disease detection — Harnessing the Web for public health surveillance. N Engl J Med 2009 May 21; 360:2153. (http://content.nejm.org/cgi/reprint/NEJMp0900702.pdf)

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