Estimating age-specific influenza-related hospitalization rates during the pandemic (H1N1) 2009 in Davidson Co, TN

Abstract

Background: In April 2009, a pandemic caused by a novel influenza strain, the A(H1N1)pdm09 virus, started. Few age-specific estimates of hospitalizations associated with the first year of circulation of the pandemic virus are available.

Objectives: To estimate age-specific hospitalization rates associated with laboratory-confirmed A(H1N1)pdm09 virus in Davidson County, TN, from May 2009 to March 2010.

Patients/methods: Two separate strategies were applied: capture-recapture and surveillance-sampling methods. For the capture-recapture estimates, we linked data collected via two independent prospective population-based surveillance systems: The Influenza Vaccine Effectiveness Network (Flu-VE) tested consenting county patients hospitalized with respiratory symptoms at selected hospitals using real-time reverse transcriptase polymerase chain reaction (rRT-PCR); the Emerging Infections Program identified county patients with positive influenza tests in all area hospitals. For the surveillance-sampling estimates, we applied the age-specific proportions of influenza-positive patients (from Flu-VE) to the number of acute respiratory illness hospitalizations obtained from the Tennessee Hospital Discharge Data system.

Results: With capture-recapture, we estimated 0·89 (95% CI, 0·72-1·49), 0·62 (0·42-1·11), 1·78 (0·99-3·63), and 0·76 (0·50-1·76) hospitalizations per 1000 residents aged < 5, 5-17, 18-49, and ≥ 50 years, respectively. Surveillance-sampling estimated rates were 0·78 (0·46-1·22), 0·32 (0·14-0·69), 0·99 (0·64-1·52), and 1·43 (0·80-2·48) hospitalizations per 1000 residents aged <5, 5-17, 18-49, and ≥ 50 years, respectively. In all age-groups combined, we estimated approximately 1 influenza-related hospitalization per 1000 residents.

Conclusions: Two independent methods provided consistent results on the burden of pandemic virus in Davidson County and suggested that the overall incidence of A(H1N1)pdm09-associated hospitalization was 1 per 1000 county residents.

Figure 1

Estimated hospitalization rates associated with A(H1N1)pdm09 virus during the pandemic (H1N1) 2009 estimated by capture–recapture and surveillance‐sampling methods in Davidson County residents aged <5, 5–17, 18–49, and ≥50 years.

Figure 2

Proportion of hospitalized patients tested positive for A(H1N1)pdm09 during the pandemic (H1N1) 2009 among the total Davidson County residents aged <5, 5–17, 18–49, ≥50 years with symptoms of acute respiratory illnesses tested for A(H1N1)pdm09 through Flu‐VE from May 2009 to March 2010.