Lessons learned from identifying clusters of severe acute respiratory infections with influenza sentinel surveillance, Bangladesh, 2009–2020

BACKGROUND: We explored whether hospital‐based surveillance is useful in detecting severe acute respiratory infection (SARI) clusters and how often these events result in outbreak investigation and community mitigation. METHODS: During May 2009–December 2020, physicians at 14 sentinel hospitals prospectively identified SARI clusters (i.e., ≥2 SARI cases who developed symptoms ≤10 days of each other and lived <30 min walk or <3 km from each other). Oropharyngeal and nasopharyngeal swabs were tested for influenza and other respiratory viruses by real‐time reverse transcriptase‐polymerase chain reaction (rRT‐PCR). We describe the demographic of persons within clusters, laboratory results, and outbreak investigations. RESULTS: Field staff identified 464 clusters comprising 1427 SARI cases (range 0–13 clusters per month). Sixty percent of clusters had three, 23% had two, and 17% had ≥4 cases. Their median age was 2 years (inter‐quartile range [IQR] 0.4–25) and 63% were male. Laboratory results were available for the 464 clusters with a median of 9 days (IQR = 6–13 days) after cluster identification. Less than one in five clusters had cases that tested positive for the same virus: respiratory syncytial virus (RSV) in 58 (13%), influenza viruses in 24 (5%), human metapneumovirus (HMPV) in five (1%), human parainfluenza virus (HPIV) in three (0.6%), adenovirus in two (0.4%). While 102/464 (22%) had poultry exposure, none tested positive for influenza A (H5N1) or A (H7N9). None of the 464 clusters led to field deployments for outbreak response. CONCLUSIONS: For 11 years, none of the hundreds of identified clusters led to an emergency response. The value of this event‐based surveillance might be improved by seeking larger clusters, with stronger epidemiologic ties or decedents.