Estimating the undetected burden of influenza hospitalizations in children

Abstract

During the 2004-2005 influenza season two independent influenza surveillance systems operated simultaneously in three United States counties. The New Vaccine Surveillance Network (NVSN) prospectively enrolled children hospitalized for respiratory symptoms/fever and tested them using culture and RT-PCR. The Emerging Infections Program (EIP) and a similar clinical-laboratory surveillance system identified hospitalized children who had positive influenza tests obtained as part of their usual medical care. Using data from these systems, we applied capture-recapture analyses to estimate the burden of influenza related-hospitalizations in children aged<5 years. During the 2004-2005 influenza season the influenza-related hospitalization rate estimated by capture-recapture analysis was 8.6/10,000 children aged<5 years. When compared to this estimate, the sensitivity of the prospective surveillance system was 69% and the sensitivity of the clinical-laboratory based system was 39%. In the face of limited resources and an increasing need for influenza surveillance, capture-recapture analysis provides better estimates than either system alone.

Fig. 1

Capture–recapture analysis using two independent sources. Peterson's estimator of N (total cases), =n1×n2/m2. Peterson's estimate implies that the estimated number of cases missed by both systems () equals (b×c)/(a); where b was the number of enrolled cases by EIP/EIP-like only, c was the number of enrolled cases by NVSN only, and a represented the number of matched cases (m2).

Fig. 2

Capture–recapture estimates. Influenza hospitalizations in children aged <5 years in three US counties, 2004–2005 influenza season.

Fig. 3

Influenza hospitalization rates in children aged <5 years by surveillance system and capture–recapture estimates in three US counties, 2004–2005 influenza season.