Epidemiological update: Novel influenza A virus A(H7N9) in China

On 31 March 2013, the Chinese health authorities announced that they identified a novel influenza A(H7N9) virus in three seriously ill patients.

As of 8 April 2013, 24 human cases of infection with influenza A(H7N9) virus have been reported in four provinces of China: Shanghai (11), Anhui (2), Jiangsu (8) and Zhejiang (3). Of these patients, seven people have died, fourteen show severe and three mild symptoms. No epidemiological link has been identified among the patients. 

More than 650 close contacts of the confirmed cases are being closely monitored. There are reports of a small family cluster of disease around the first patient, but this has not been confirmed by laboratory data.

The source of these infections and the mode of transmission are yet to be determined. This is the first time that human infection with influenza A(H7N9) virus has been identified.

The Chinese health authorities are responding to this public health event by enhanced surveillance, epidemiological and laboratory investigation and contact tracing. The animal health sector has intensified investigations into the possible sources and reservoirs of the virus. The authorities reported to the World Organisation for Animal Health (OIE) that A(H7N9) was detected in samples from pigeons and chickens and in environmental specimens from three markets in Shanghai. These markets have been closed and the live poultry were culled.

The influenza A viruses from the first three patients were non-subtypeable and were sent to the WHO Influenza Collaborating Centre at the Chinese Centre for Disease Control and Prevention (CCDC). The genetic comparison indicated that these cases were caused by a novel reassortant avian influenza virus with avian origin genes from both A(H7N9) and A(H9N2). No similar viruses have been seen before. In addition, A(H7N9) differs from A(H7) and A(H9) viruses that have been seen previously in Europe.

No vaccine is currently available for this subtype of the influenza virus. Preliminary test results suggest that the virus is susceptible to the neuraminidase inhibitors (oseltamivir and zanamivir).

At this time, there is no evidence of on-going human-to-human transmission. More sporadic cases are expected to be reported. The risk of disease spread to Europe is considered low, although individual cases coming from China cannot be ruled out.

Table 1: Distribution of influenza A(H7N9) cases by date of onset of symptoms, age, gender, status and province in China, as of 8 April 2013 (n=24)

Figure 1: Distribution of influenza A(H7N9) cases in China by week of onset of symptoms, as of 8 April 2013 (n=24)

Figure 2: Distribution of cumulative number of confirmed influenza A(H7N9) cases by province in China, 19 February to 8 April 2013