In March 2013, Chinese authorities announced the identification of a novel reassortant A(H7N9) influenza virus in patients in eastern China.
Since then, human cases have continued to be reported, and as of 7 February 2014 (Figure 1), there have been 308 laboratory-confirmed cases: Zhejiang (122), Guangdong (54), Shanghai (42), Jiangsu (36), Fujian (19), Hunan (7), Jiangxi (5), Henan (4), Anhui (4) ,Beijing (3), Shandong (2), Hebei (1), Guangxi (2), Guizhou (1), Hong Kong (4) and Taiwan (2). In addition, the virus has been detected in one asymptomatic case in Beijing.
The below map demonstrates the emergence of the cases in the two peaks of the disease (Figure 2).
Figure 1: Distribution of confirmed A(H7N9) cases by place of reporting, week 08/2013 to 06/2014, China (n=308)
Since 15 October 2013, 173 cases were reported from: Zhejiang (76), Guangdong (53), Fujian (14), Jiangsu (9), Shanghai (8), Hunan (4), Beijing (1), Guangxi (2), Guizhou (1), Taiwan (1) and Hong Kong (4).
Figure 2: Distribution of confirmed A(H7N9) cases by week* and degree of severity**, week 08/2013 to 06/2014, China (n=308)
Overall, most cases have developed severe respiratory disease. At least 63 cases are known to have died, although this information is not systematically reported to WHO or ECDC (case-fatality ratio=20.5%). The average age is 55.5 years, ranging from 2 to 91 years; 100 of 308 patients are female, with gender being unknown in five cases.
Figure 3: Distribution of confirmed A(H7N9) cases by age and gender, 31/03/2013-07/02/2014, China (n=303*)
There have been two peaks of the disease in China (Figure 2), the age and gender division in those two peaks does not differ considerably. In the first peak of the 130 cases where the age and gender was known, 70% (91) were male and 30% (39) were female. In the second peak, out of the 173 cases where the age and gender was known, 65% (112) were male and 35% (61) were female.
During the first peak, out of the 135 cases, 43 died (CFR = 32%), whereas during the second peak there have been 173 cases, of which 20 have died (CFR = 11.6%). These estimates are based on the information available at time of notification and it must be considered that the CFR among recent cases might rise due to the disease progression among them. While the data is scarce for the first peak regarding the date of hospitalisation and severity, during the second peak, 171 patients have been hospitalised and, out of these, 149 have been reported either severe or critical at the time of notification.
A few small clusters were detected but almost all cases have occurred sporadically, without obvious epidemiological links among them. While occasional human-to-human transmission in the clusters cannot be ruled out, there is no confirmed sustained human-to-human transmission. A few mild cases have been detected.
Most human A(H7N9) cases have reported contact with poultry or live bird markets. The most plausible underlying scenario is of a zoonotic avian influenza that has spread or is spreading in poultry in parts of eastern China. It is a severe threat to humans because of its severity and genetic features that have human pandemic potential.