New Study by Chinese and CDC Experts Shows H7N9 Illness Uncommon in China

May 9, 2013 – A study by CDC and Chinese public health experts published on May 8, 2013, indicates that human infections with the H7N9 virus remain very rare in China among people seeking medical care for influenza-like illness (ILI)*. This study presents some of the first available information regarding the prevalence of H7N9 among people in China.

Researchers found that only 0.03 percent of patients who sought medical care for ILI at sentinel sites in China from March 4 through April 28, 2013 were infected with the H7N9 virus. However, the study’s authors cautioned that China’s ILI surveillance network covers mostly urban areas and may not capture H7N9 infections among people living in rural areas of China.

Data for the study was collected through the “Chinese National Influenza Like Illness Surveillance Network” (CNISN), which includes 554 sentinel hospitals and 408 network laboratories operating in all 31 provinces of China.

The CNISN network tested 46,807 swabs from patients seeking health care for ILI during the study period. These swabs included 20,739 ILI surveillance specimens from 141 sentinel hospitals in 10 provinces where H7N9 cases have been detected. These provinces include Shanghai, Anhui, Jiangsu, Zhejiang, Beijing, Shandong, Henan, Fujian, Jiangxi, and Hunan. Of these 20,739 samples, only 6 (0.03%) tested positive for H7N9 infection. Among the six positive H7N9 cases, four of the patients were hospitalized for pneumonia complications. Of these four patients, three were 25-29 years old, and one was 69 years of age. Two of the six people infected with H7N9 were not admitted to a hospital.

In addition, four of the six patients with H7N9 infection reported a history of contact with live chickens or having visited a live poultry market. This is consistent with other reports out of China that most of the cases of H7N9 have reported poultry contact or exposure to environments that might be contaminated with H7N9 virus, like live bird markets, for example.

Five provinces reported one or more cases of ILI testing positive for H7N9 virus during the study period. The percent of all patient visits due to ILI increased during the study, but the proportion of ILI specimens that tested positive for flu decreased. Furthermore, there were no detections of “unsubtypable” (and therefore, novel) flu viruses from the patients tested. This suggests that the increase in the health care visits among patients with ILI may be attributed to increased health-care seeking behavior following media reports of the H7N9 outbreak. The researchers concluded that overall, H7N9 virus infection remains an uncommon cause of ILI among patients in China’s provinces reporting confirmed H7N9 cases.

The study, entitled “Monitoring avian influenza A(H7N9) virus through national influenza-like illness surveillance, China” was published in the journal Emerging Infectious Diseases (EID). This study was a collaborative effort between Chinese public health and CDC Influenza Division experts. The CDC team traveled to China to assist in investigating the H7N9 virus outbreak in response to a bi-lateral request for assistance from China.

*ILI is defined as fever (temperature of 38 degrees Celsius or greater) and cough or sore throat.