To the Editor: The Naval Health Research Center serves as theNavy hub for the Department of Defense's Global Emerging InfectionsSurveillance and Response System (GEIS), in which it monitorsinfluenza-like illness among recruit trainees of all militaryservices, military dependents, and crew members of large Navyships (population, >1000). The center works in collaborationwith the Border Infectious Disease Surveillance Project of theCenters for Disease Control and Prevention (CDC), which monitorspopulations located on the border between California and Mexico.The first two human cases of novel swine-origin influenza A(H1N1) virus (S-OIV), known as swine flu, in the United Stateswere detected through these programs.1 In the first case, anuntypeable influenza A strain was identified at a surveillancesite of the Naval Health Research Center by a new diagnosticdevice. The test results were forwarded per protocol to thestudy reference laboratory for polymerase-chain-reaction (PCR)confirmation and were subsequently forwarded to the CDC foridentification by sequencing. In the second case, a sample thatwas obtained at a border surveillance site was found to containan untypeable influenza A strain on PCR testing at the center.Further characterization by PCR assay and electrospray ionizationmass spectrometry indicated a swine-origin virus, and sequencedata that were sent to the CDC revealed that the viruses inthe two samples were identical. In response, surveillance activitiesof all programs were enhanced to include increased samplingrates, more clinical sites, decreased turnaround time in thelaboratory, and rapid influenza testing with the use of QuickVueInfluenza A+B (Quidel).
From April 20 through May 30, 2009, the center processed 3066specimens with the use of a real-time reverse-transcriptasePCR (RT-PCR) assay,2 which revealed 273 confirmed cases of S-OIV(8.9%), 18 cases of H1N1 seasonal influenza (0.6%), and 31 casesof H3N2 influenza (1.0%) (Figure 1). All suspected cases ofS-OIV were confirmed with the use of the CDC's S-OIV assay.2All specimens were collected from patients with influenza-likeillness who met the CDC's guidelines for screening. Rapid-testresults for 767 patients during this influenza season were availablefor comparison and were positive for 20 of 39 patients who hadpositive results for S-OIV on RT-PCR assay (sensitivity, 51%;95% confidence interval [CI], 35 to 67), for 12 of 19 patientswho had positive results for H1N1 seasonal influenza on RT-PCR(sensitivity, 63%; 95% CI, 39 to 82), and for 6 of 19 of patientswho had positive results for H3N2 influenza on RT-PCR (sensitivity,31%; 95% CI, 14 to 57). The specificity of the test, as comparedwith that of RT-PCR, was 99% in all cases.
Figure 1. Results of RT-PCR Analyses of 3066 Specimens, According to Influenza A Subtype.
All analyses were performed from April 20 through May 30, 2009, at the Naval Health Research Center. Only specimens for which collection dates were available are included. H1N1 denotes H1N1 seasonal influenza, RT-PCR reverse-transcriptase–polymerase chain reaction, and S-OIV swine-origin influenza A (H1N1) virus.
Uyeki et al. described the poor sensitivity of the Quidel test(mean, 27%; range, 19 to 32) for influenza during the 2007–2008season.3 During the 2008–2009 season, we also found alow sensitivity of the test for seasonal influenza strains thatwere cocirculating with S-OIV, although Uyeki et al. did notdifferentiate among subtypes. The performance of current influenzarapid antigen tests in diagnosing S-OIV is unknown.4 Our findingssuggest that rapid-test sensitivity may vary according to theinfluenza A subtype. Further investigation is needed to confirmthis finding and evaluate possible explanations.
The identification of S-OIV in Southern California highlightsthe use of multiple, complementary surveillance systems in detectingand validating unusual disease activity. Specifically, the earlydetection of two epidemiologically unrelated cases of S-OIVin two surveillance systems was instrumental in quickly alertingpublic health officials and mobilizing an effective response.
S-OIV continues to cocirculate with seasonal influenza strainsbut may be differentially detected by rapid influenza tests.This finding has implications for the diagnosis and treatmentof patients with influenza-like illness now and during the nextinfluenza season. As seasonal and zoonotic influenza virusescontinue to drift and shift, we must continuously assess thesensitivity and specificity of available diagnostic tests.
Dennis J. Faix, M.D., M.P.H. Naval Health Research Center San Diego, CA dennis.faix{at}med.navy.mil
Sterling S. Sherman, M.D., M.P.H. Naval Medical Center San Diego, CA
Steven H. Waterman, M.D., M.P.H. Centers for Disease Control and Prevention San Diego, CA
Data that are presented were collected during ongoing surveillancestudies of the 2008–2009 influenza season and activitiesrelated to the S-OIV outbreak during April and May 2009, allof which were supported by the Department of Defense's GEIS,the Department of State, and the Department of Health and HumanServices. The Department of the Navy supported outbreak-responseactivities during May 2009.
Dr. Waterman reports receiving grant support from Sanofi Pasteur.No other potential conflict of interest relevant to this letterwas reported.
This letter (10.1056/NEJMc0904264) was published on June 29,2009, at NEJM.org.
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