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 Chinese investigators are still trying to determine the exact cause of the outbreak.

The investigation has centered 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, who evidently never worked directly with the SARS virus, developed the disease in late March and mid-April. The outbreak was reported on April 22 and the institute was closed a day later.

Although preliminary findings in the investigation have yet to identify a single infectious source or single procedural error at the institute, investigators have serious concerns about biosafety procedures there. Both the WHO and Chinese authorities view the occurrences of laboratory-associated infections very seriously.

The WHO has urged all member states to review the biosafety practices of institutions and laboratories working with SARS coronavirus.

During and after the SARS outbreak of 2003, a large number of specimens were collected from possible human cases, animals and the environment.

These specimens, which may contain live SARS coronavirus, are still kept in various laboratories around the world. Some of them are stored in laboratories at an inappropriate containment level.

SARS coronavirus has also been propagated in reference and research laboratories, and distributed to other laboratories for research purposes. Research using live and inactivated SARS coronavirus is being conducted in many laboratories.

(World Health Organization, http://www.who.int/csr/don/2004_05_18a/en/ May 18, 2004)