People infected with H7N9 can shed virus for 3 weeks, potentially infecting others

On March 31, 2013, the China Health and Planning Commission notified WHO of three human infections in Shanghai and Anhui with a novel influenza virus characterised as avian influenza A H7N9 (illness onset between Feb 19 and March 15, 2013).

The severity of disease was remarkable, as was the fact that patients were from towns located 400 km apart, and had no epidemiological connection
Yu Chen and colleagues report details of their first H7N9-infected patient from Zhejiang province, and three additional cases who were identified by retrospectively testing 486 patients admitted to hospital with pneumonia between March 7 and April 9, 2013. Combined with those described by Gao and coworkers, Chen and colleagues' findings show a disease profile characterised by bilateral pneumonia progressing to acute respiratory distress syndrome, and multiorgan dysfunction that resembles human infections with highly pathogenic avian influenza A H5N1 virus or severe seasonal or pandemic influenza.

Chen and coworkers' findings add some essential pieces of information to further understanding of the emerging virus. First, viral RNA shedding can be prolonged (up to 11, 17, and 20 days in three patients), which might indicate absent previous immunity or the presence of comorbidities, or both. 

Second, the virus might not be detected in upper-respiratory-tract specimens, such as throat swabs, but could still be readily detectable in lower-respiratory-tract specimens, which, as for H5N1 infections, emphasises the importance of collecting lower-respiratory-tract specimens (if possible) when H7N9 infection is suspected in severely ill patients.

Third, although data for only two patients are presented, the concentrations of some cytokines and chemokines and viral loads were higher in a patient who died than in one who was recovering at the time of writing.3

Such findings have also been noted in H5N1 and the 2009 H1N1 infections.

Finally, whereas most reported cases of H5N1 infection were in previously healthy children or young adults (median age 27 years in China), the seven patients infected with H7N9 virus reported so far had comorbidities, and the age profile of cases is skewed (median age 63 years).

Most known patients were identified when they were admitted to hospital with severe respiratory disease, and the retrospective case-finding described by Chen and colleagues shows that diagnosis might have been missed, even in patients with severe disease. Besides surveillance in patients with mild respiratory symptoms, which could easily overwhelm diagnostic capacities, targeted serological studies could be considered to test whether seroprevalence is higher in people who regularly visit live markets than in those who do not regularly visit markets, and possibly look for other sources of infection.

Almost a third of notified patients up to now did not have any recognised exposure to poultry, and only a small proportion of birds tested have been positive for the virus.

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