Epidemiological update of 13 April: avian influenza A(H7N9) virus 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 12 April 2013, forty cases of human infection with influenza A(H7N9) have been reported from four provinces in eastern China (20 in Shanghai, 12 in Jiangsu, six in Zhejiang and two in Anhui) since 31 March 2013, with onset of disease between 19 February and 5 April 2013. The date of disease onset is currently unknown for three patients. Eleven patients died (case-fatality ratio=27%), 27 are severe cases and three are mild cases. Among the severe cases, patients are reported to be in a stable condition. The median age is 65 years with a range between 4 and 87 years; 12 of them are females.

No epidemiological link among cases has been identified to date. More than 700 close contacts of the confirmed cases are being closely monitored. There are reports of a potential small family cluster of disease around the first case, but this has not been  confirmed by laboratory data. In Jiangsu, investigation is on-going regarding a contact of an earlier confirmed case who developed symptoms of illness.

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 virus has also been detected in 14 positive samples from five additional live bird markets in Jiangsu, Anhui and Zhejiang. It is currently unknown from which type of birds the samples have been taken.

The influenza A viruses from the first three cases 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 and 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 and there has not been further geographical spread. 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.

Figure 1: Distribution of influenza A(H7N9) cases by date of onset, age, gender, status and province, China, as of 12 April 2013 (n=40)

Figure 2: Distribution of influenza A(H7N9) cases by week of onset of symptoms, China, as of 12 April 2013 (n=40)

Figure 3: Distribution of cumulative number of influenza A(H7N9) cases by province, China, 19 February – 12 April 2013