It has been more than three weeks since the last case was placed in isolation in China's latest SARS outbreak, prompting the World Health Organization to declare that the chain of human-to-human transmission appears to have been broken.

However, WHO experts and the Chinese authorities are still trying to determine the exact cause of the outbreak. The investigation has centered primarily on the National Institute of Virology in Beijing where experiments using live and inactivated SARS coronavirus have been carried out. Two researchers at the Institute developed SARS in late March and mid-April. The outbreak was reported on April 22 and the Institute was closed a day later.

Preliminary findings in the investigation have yet to identify a single infectious source or single procedural error at the Institute–and it is conceivable that an exact answer may never be arrived at. Neither of the researchers is known to have directly conducted experiments using live SARS coronavirus. However, investigators have serious concerns about biosafety procedures at the Institute–including how and where procedures using SARS coronavirus were carried out, and how and where SARS coronavirus samples were stored.

WHO and Chinese authorities view with concern the occurrence of laboratory-associated SARS cases. WHO urges all Member States to view this latest outbreak as an opportunity to review the biosafety practices of institutions and laboratories working with SARS coronavirus.