The H7N9 influenza virus has killed over a third of patients who were hospitalized with the infection, experts wrote in The Lancet.

The study, conducted by researchers at the Chinese Center for Disease Control and Prevention (CDC), in Beijing, China, and the University of Hong Kong, provides the first estimates of the severity profile of the virus.

Data on hospital admissions associated with H7N9 infection were gathered and examined for the purpose of the study. In order to estimate the likelihood of fatality for patients who had been admitted to hospital and the risk of death in symptomatic cases, the experts also used surveillance data.

H7N9, the avian influenza that is in circulation in China at the moment, has a lower fatality risk than H5N1, an avian influenza virus that appeared in 2003, the authors said. However, the death rate from H7N9 infection is higher than the H1N1 pandemic influenza from 2011.

"H5N1 had a fatality risk of around 60% for patients admitted to hospital, whereas pandemic H1N1 killed 21% of patients with the virus who were admitted to hospital," the experts said.

After looking at data from China's sentinel surveillance network, and based on assumptions of how many infected people are likely to seek medical care, the authors explained that between 0.16% and 2.8% of all people with symptomatic H7N9 infection are at risk of death.

Due to the challenges of correctly calculating how many patients are infected with H7N9 but only have mild symptoms, there is a wide discrepancy between these estimates.

The researchers explained:

"Assessing the severity profile of human infections is vitally important in the management and treatment of any infectious disease outbreak. Although previous clinical case series have focused on the potential for avian influenza H7N9 virus infection to cause severe illness, we have estimated that mild cases might have occurred."

"Our results thus support continued vigilance and sustained intensive control efforts against the virus to minimize risk of human infection, which is greater than previously recognized," they added.

In a second report, published simultaneously, the same group of investigators analyzed the epidemiological characteristics of H7N9 and compared them with older H5N1 viruses.

There is a possibility that H7N9 will reappear later this year, the authors warned, and public health officials and health care workers need to be ready.

Men appear to have been more vulnerable to infection from both viruses, especially in urban areas. The researchers believe that a leading risk factor for infection is the handling of infected poultry.

The report also highlighted some notable differences between infections caused by the two viruses. For example, disease progression was experienced more rapidly with H5N1 infections, and the risk of dying for hospitalized patients with this virus was almost two times the risk for H7N9.

"The incubation period of H7N9 was estimated at 3.3 days on average, lower than previous estimates, and directly informing quarantine policies," according to the report.

The authors concluded:

"The warm season has now begun in China, and only one new laboratory-confirmed case of H7N9 in human beings has been identified since May 8, 2013. If H7N9 follows a similar pattern to H5N1, the epidemic could reappear in the autumn.

This potential lull should be an opportunity for discussion of definitive preventive public health measures, optimization of clinical management, and capacity building in the region in view of the possibility that H7N9 could spread beyond China's borders."

The investigation is part of an ongoing collaborative effort between the CDC and the University of Hong Kong's School of Public Health.

The scientists' goal is to define the epidemiology of influenza A (H7N9) in real time.

A recent study revealed that the H7N9 bird flu virus may be human transmissible through direct contact as well as through airborne exposure.

A different report said that a vaccine which provides wider protection against multiple strains of the bird flu virus is currently being worked on.

Written by Sarah Glynn