Highly pathogenic avian influenza A (HPAI) subtype H5N1 has caused family case clusters, mostly in
Southeast Asia, that could be due to human-to-human transmission. Should this virus, or another
zoonotic influenza virus, gain the ability of sustained human-to-human transmission, an influenza
pandemic could result. We used statistical methods to test whether observed clusters of HPAI (H5N1)
illnesses in families in northern Sumatra, Indonesia, and eastern Turkey were due to human-to-human
transmission. Given that human-to-human transmission occurs, we estimate the infection secondary
attack rates (SARs) and the local basic reproductive number, R0. We find statistical evidence of humanto-
human transmission (p = 0.009) in Sumatra but not in Turkey (p = 0.114). For Sumatra, the estimated
household SAR was 29% (95% confidence interval [CI] 15–51). The estimated lower limit on the local R0
was 1.14 (95% CI 0.61–2.14). Effective HPAI (H5N1) surveillance, containment response, and field
evaluation are essential to monitor and contain potential pandemic strains.

Detecting Human-to-Human Transmission of Influenza A (H5N1).pdf