That there is no evidence of human-to-human transmission of the H7N9 bird flu virus is reassuring, yet government health departments and experts must remain vigilant, since it is still not known how the two men who died in Shanghai became infected.

The National Health and Family Planning Commission said it wasn't clear how the two men who died, and a women in Anhui province who is in critical condition, had contracted the virus, but there was no sign that any of the three had contracted the disease from each other, and no sign of infection in the people who had closest contact with them.

However, there has been little research on the virus, since this is the first time it has been detected in humans, and there is no vaccine against it. Therefore a thorough investigation needs to be conducted to find out how these three people became infected.