Background On April 15 and April 17, 2009, novel swine-origininfluenza A (H1N1) virus (S-OIV) was identified in specimensobtained from two epidemiologically unlinked patients in theUnited States. The same strain of the virus was identified inMexico, Canada, and elsewhere. We describe 642 confirmed casesof human S-OIV infection identified from the rapidly evolvingU.S. outbreak.
Methods Enhanced surveillance was implemented in the UnitedStates for human infection with influenza A viruses that couldnot be subtyped. Specimens were sent to the Centers for DiseaseControl and Prevention for real-time reverse-transcriptase–polymerase-chain-reactionconfirmatory testing for S-OIV.
Results From April 15 through May 5, a total of 642 confirmedcases of S-OIV infection were identified in 41 states. The agesof patients ranged from 3 months to 81 years; 60% of patientswere 18 years of age or younger. Of patients with availabledata, 18% had recently traveled to Mexico, and 16% were identifiedfrom school outbreaks of S-OIV infection. The most common presentingsymptoms were fever (94% of patients), cough (92%), and sorethroat (66%); 25% of patients had diarrhea, and 25% had vomiting.Of the 399 patients for whom hospitalization status was known,36 (9%) required hospitalization. Of 22 hospitalized patientswith available data, 12 had characteristics that conferred anincreased risk of severe seasonal influenza, 11 had pneumonia,8 required admission to an intensive care unit, 4 had respiratoryfailure, and 2 died. The S-OIV was determined to have a uniquegenome composition that had not been identified previously.
Conclusions A novel swine-origin influenza A virus was identifiedas the cause of outbreaks of febrile respiratory infection rangingfrom self-limited to severe illness. It is likely that the numberof confirmed cases underestimates the number of cases that haveoccurred.
Source Information
The members of the writing group (Fatimah S. Dawood, M.D., Epidemic Intelligence Service, Office of Workforce and Career Development; and Seema Jain, M.D., Lyn Finelli, Dr.P.H., Michael W. Shaw, Ph.D., Stephen Lindstrom, Ph.D., Rebecca J. Garten, Ph.D., Larisa V. Gubareva, M.D., Ph.D., Xiyan Xu, M.D., Carolyn B. Bridges, M.D., and Timothy M. Uyeki, M.D., M.P.H, M.P.P., Influenza Division, National Center for Immunization and Respiratory Diseases — all at the Centers for Disease Control and Prevention, Atlanta) assume responsibility for the overall content and integrity of the article. This article (10.1056/NEJMoa0903810) was published on May 7, 2009, and updated on May 22, 2009, at NEJM.org. It will appear in the June 18 issue of the Journal.
Address reprint requests to Dr. Dawood at the Influenza Division, Centers for Disease Control and Prevention, 1600 Clifton Rd. NE, MS A-32, Atlanta, GA 30333, or at fdawood{at}cdc.gov; or to Dr. Shaw at the Influenza Division, Centers for Disease Control and Prevention, 1600 Clifton Rd. NE, MS G-16, Atlanta, GA 30333, or at mshaw1{at}cdc.gov.
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