Human to Human
Human-to-human transmission of influenza A (H5N1) has been suggested in several household clusters and in one case of apparent child-to-mother transmission (Table 3). Intimate contact without the use of precautions was implicated, and so far no case of human-to-human transmission by small-particle aerosols has been identified. In 1997, human-to-human transmission did not apparently occur through social contact, and serologic studies of exposed health care workers indicated that transmission was inefficient9 (Table 2). Serologic surveys in Vietnam and Thailand have not found evidence of asymptomatic infections among contacts (Table 2). Recently, intensified surveillance of contacts of patients by reverse-transcriptase–polymerase-chain-reaction (RT-PCR) assay has led to the detection of mild cases, more infections in older adults, and an increased number and duration of clusters in families in northern Vietnam, findings suggesting that the local virus strains may be adapting to humans. However, epidemiologic and virologic studies are needed to confirm these findings. To date, the risk of nosocomial transmission to health care workers has been low, even when appropriate isolation measures were not used (Table 2). However, one case of severe illness was reported in a nurse exposed to an infected patient in Vietnam.