A new study suggests there have been multiple clusters of human-to-human transmission in recent outbreaks of the bird flu strain H7N9.

There were around 400 human cases of H7N9 influenza and 177 deaths in 2013 and 2014, all of them in China. Most patients are believed to have caught the infection from close contact with birds, but the virus’s ability to spread between humans has been uncertain.

In a study published in Emerging Infectious Diseases, scientists from Imperial College London studied data from these outbreaks and used statistical methods to estimate how transmissible the virus is.

The results suggest that around 70 cases were caused by an infection spread between people. However, the virus cannot spread easily enough in humans to cause sustained transmission at the level required for a pandemic. 

The number of people one infected person will pass on the infection to, on average, is called the basic reproductive number. If the value is less than one, an outbreak would be expected to die out; while a value greater than one suggests an outbreak would grow.

In the outbreaks studied, the reproductive number ranged from 0.06 to 0.35. This means that once the virus infects a person, there is only a small risk of that person passing it to someone else.

The researchers warned that H7N9 poses a continuing threat, and authorities must be vigilant in case the virus becomes more transmissible.

Dr Steven Riley, senior author of the study, from the Medical Research Council Centre for Outbreak Analysis and Modelling at Imperial, said: “This study shows that H7N9 is currently short of the critical level of transmissibility required to cause a pandemic. But even if the reproductive number is less than one, clusters of human transmission can occur.

“In Zhejiang, the reproductive number increased between the first wave in 2013 and the second wave in 2014. We have to keep an eye on further outbreaks to see how the virus is evolving.”

The study also looked at the effectiveness of closing live bird markets, which are thought to be the main source of infections for humans. Closing the markets for short periods had little effect on the risk, but longer closures appeared to be more effective.

Dr Adam Kucharski, who worked on the study at Imperial before moving to the London School of Hygiene & Tropical Medicine, said: “Our findings suggest that prompt market closures for a sustained period can substantially reduce the number of infections.”

The study was funded by the Wellcome Trust, the Medical Research Council, the National Institute of General Medical Sciences, the EU Seventh Framework Programme, the Fogarty International Center and the Research and Policy for Infectious Disease Dynamics programme.

Reference: Kucharski AJ, Mills HL, Donnelly CA, Riley S. Transmission potential of influenza A(H7N9) virus, China, 2013–2014. Emerg Infect Dis. 2015 May. http://dx.doi.org/10.3201/eid2105.141137