Epidemiology

As of August 31, 2016, mainland China had reported a total of 775 laboratory-confirmed human infections with A(H7N9) virus from 16 provinces and three municipalities during the four epidemics. In addition, travelers to mainland China accounted for 23 human cases of A(H7N9) virus infection, including four deaths; these infections were detected in Hong Kong (16 cases), Taiwan (four), Canada (two), and Malaysia (one).

Among 314 counties in China that reported at least one human A(H7N9) virus infection, 224 (71%) reported ≤2 infections. Most (83%) infections were reported in five eastern or southeastern coastal provinces. Whereas most infections in the first epidemic were identified during March–April 2013, the majority of infections identified in the subsequent three epidemics occurred during November–April of 2013–2014, 2014–2015, and 2015–2016 (Figure).

Among the 775 total reported infections, 659 (85%) patients reported exposure to live poultry in the 2 weeks preceding illness onset, including live-poultry markets (376 patients, 57%), backyard poultry (115, 17%), or both (120, 18%); and in other settings (48, 7%) (Table). Median age did not significantly differ between persons infected in the fourth epidemic (58 years) compared with the previous three epidemics (57 years). Twenty-five (3%) persons reported living with, working with, or having another epidemiologic link to a person infected with influenza A(H7N9) virus.

Among all 775 infections in the four epidemics, 55 (7%) were associated with 26 clusters (i.e., at least two epidemiologically linked infections), including 23 clusters of two infections each, and three clusters of three infections each. Most (23, 88%) clusters included family members only, and three involved nosocomial transmission (3,4). Among the index patients in the 26 clusters, 25 (96%) had a history of live poultry exposure in the 2 weeks before illness onset; secondary infections (29) in clusters resulted from possible human-to-human transmission (18), exposure to a common infectious source (three), or undetermined exposures (eight). The proportion of persons identified within clusters in the fourth epidemic was similar to the proportion in the three previous epidemics combined (10% compared with 7%, p = 0.16). There was no evidence of tertiary transmission in any cluster.

Fewer A(H7N9) infections were reported during the fourth epidemic (n = 118) than in the first (134), second (304), or third (219) epidemics. The epidemic period during which persons developed illness in the fourth epidemic (interquartile range = 73 days) was more than four times as long as that noted during the first epidemic (15 days), twice as long as the second (35 days), and more than one and a half times as long as the third epidemic (43 days). More than half of infections in the fourth epidemic were reported from two adjacent provinces located on the southeast coast of China; however, one province (Liaoning) and one municipality (Tianjin City) each reported their first A(H7N9) virus infection in the fourth epidemic, indicating spread of the virus to new areas. The percentage of A(H7N9) virus–infected persons living in rural areas in the fourth epidemic was higher than in the three previous epidemics combined (54% compared with 42%; p = 0.01).

Since April 2013, the Ministry of Agriculture in China has published surveillance data on poultry samples tested for the presence of A(H7N9) virus. As of September 1, 2016, a total of 233 positive samples in 16 provinces were detected. All samples were from live-poultry markets, except one from a farmer’s free-range backyard flock.