Emergency Department Visits for Influenza A(H1N1)pdm09, Davidson County, Tennessee, USA

To determine the number of emergency department visits attributable to influenza A(H1N1)pdm09 in Davidson County, Tennessee, USA, we used active, population-based surveillance and laboratory-confirmed influenza data. We estimated ≈10 visits per 1,000 residents during the pandemic period. This estimate should help emergency departments prepare for future pandemics.

departments for acute respiratory infection (ARI) or fever/ feverishness for <14 days during May 1, 2009-March 31, 2010. Nasal and throat swabs were tested for infl uenza with reverse transcription PCR (RT-PCR) by using primers and probes provided by the Centers for Disease Control and Prevention (Atlanta, GA, USA) (8). Specimens were classifi ed as A(H1N1)pdm09 virus if results were positive on both pandemic subtyping assays (pandemic A and pandemic H1) or positive for infl uenza A, negative for seasonal subtypes H1 and H3, and positive on 1 pandemic subtyping assay.
We obtained the number of emergency department visits associated with ARI or fever (International Classifi cation of Diseases, Ninth Revision, Clinical Modifi cation, codes 381-382, 460-466, 480-487, 490-493, 786, and 780.6) from the Tennessee Hospital Discharge Data System (HDDS) (9), which is required to include a record of every hospital-based health care encounter. We combined data from Flu-VE RT-PCRs, infl uenza test results obtained clinically in the surveillance emergency departments, and HDDS discharge diagnoses to calculate age-specifi c visit rates attributable to infl uenza A(H1N1)pdm09. We used 2 epidemiologic methods: surveillance sampling and capture-recapture.
For surveillance sampling, we enrolled 826 (52%) of 1,589 eligible patients in the Flu-VE study who had visited surveillance emergency departments; 88 (11%) had positive RT-PCR results for A(H1N1)pdm09 virus (Figure). We divided the pandemic period into 3 intervals according to prevalence of A(H1N1)pdm09 among Flu-VE participants: prepeak (May-July 2009), peak (August-November 2009), and postpeak (December 2009-March 2010). Within each period, we assumed that the proportion of ARI-or fever-associated visits caused by A(H1N1)pdm09 virus among enrolled county residents was the same as that for such emergency department visits among all county residents. Estimated infl uenza A(H1N1)pdm09-associated emergency department visits were thus calculated by multiplying age-and time-specifi c counts of total county ARI-or fever-associated emergency department visits by these proportions (Table 1). We divided age-specifi c counts by age-specifi c county population estimates for July 2009 (10) and calculated rates per 1,000 residents ( Table 2). We used the binomial Wilson method to calculate 95% CIs for the proportions of ARI-or fever-associated emergency department visits caused by A(H1N1)pdm09 virus.
We developed a capture-recapture model (11) by linking 2 independent data sources for infl uenza testing from the same population: the Flu-VE RT-PCRs, performed in a research laboratory and not reported to patients or clinicians, and infl uenza tests performed as routine care in the surveillance emergency departments. Unlike the research laboratory tests, not all clinical tests included  (7), and >50 (0). Using the nearly unbiased estimator equation, we calculated 572, 1,000, 528, and 90 surveillance emergency department visits for infl uenza A(H1N1)pdm09 for each age group, respectively. HDDS data indicated that 62.3%, 48.4%, 18.3%, and 14.2% of ARI-or fever-associated emergency department visits among county residents <5, 5-17, 18-49, and >50 years of age, respectively, occurred in surveillance emergency departments. We calculated the total number of infl uenza A(H1N1)pdm09-associated emergency department visits by county residents by dividing the number of infl uenza A(H1N1)pdm09-associated visits to surveillance emergency departments by the agespecifi c proportions above. To estimate rates, we divided estimated infl uenza A(H1N1)pdm09 visits by age-specifi c county populations for July 2009 (10) and multiplied by 1,000, yielding rates comparable to those obtained by the surveillance sampling method ( Table 2). We calculated 95% CIs for capture-recapture estimates by using a bias-corrected bootstrap method (14). Because no persons >50 years of age were identifi ed by both surveillance systems, 95% CIs for this group and the entire population could not be calculated.

Conclusions
Using 2 epidemiologic techniques for calculating rates, we found that ≈1% of the Davidson County, Tennessee, population had visited an emergency department for infl uenza A(H1N1)pdm09 during the fi rst year of virus circulation. The study has several limitations. The reported rates are dependent on the sensitivity and specifi city of infl uenza tests. Delays in seeking care could have resulted in some infl uenza A(H1N1)pdm09 cases being undetectable, and if so, rates reported here would underestimate true rates of infl uenza A(H1N1)pdm09-attributable emergency department visits. Because active surveillance activities did not infl uence the possibility of infl uenza identifi cation through routine emergency department care, the independence of these systems was assumed for capture-recapture calculations. However, this assumption could have been violated in some instances, for example  if infl uenza viral load varied substantially among persons and higher viral loads increased the likelihood of detection by both systems. In this scenario, our method would underestimate the true number of emergency department visits (by increasing the number of matched cases). The proportions of ARI-and fever-associated emergency department visits for A(H1N1)pdm09 virus infection were extrapolated from surveillance emergency departments to the entire county population. If this proportion were higher (or lower) in the surveillance emergency departments than in other emergency departments, our rates would overestimate (or underestimate) true rates. Additionally, the small number of cases detected in adults >50 years of age precluded further age stratifi cation among older adults. Because this study was conducted in an urban US county with high accessibility to emergency departments, we advise caution when extrapolating our estimates directly to other populations. A modern infl uenza pandemic of mild severity can quickly cause large surges in emergency department visits. To minimize emergency department overcrowding and to maximize effi cient use of resources, long-term preparation for these surges is vital. The high number of emergency department visits during the pandemic also illustrates the large effect a novel infl uenza stain can have on an unvaccinated, susceptible population and highlights the need for continued infl uenza vaccine development and use.