Russian authorities investigate suspected plague case linked to lab worker's death

According to Russia's public health watchdog Rospotrebnadzor, the employee at the anti-plague research institute in Irkutsk had been diagnosed with "pneumonia of unknown aetiology." The situation in the cities of Irkutsk and Shelekhov was "stable," and measures have been implemented in response to the case, it said in a statement on Sunday.

Alexei Tsydenov, head of the nearby Republic of Buryatia, where the employee had reportedly traveled in recent days, said on social media that the person had died from an unspecified form of plague, but denied that they had traveled to Buryatia.

CNBC has not been able to independently verify the reports. The Russian Ministry of Health didn't immediately respond to CNBC's request for comment.

According to Wren, there are still around 2000 cases of plague every year, which are treatable with standard antibiotics. "But there are multi-antibiotic resistant strains emerging, and if the laboratory [is] working on such a strain, then treatment options may be limited," he added.

A lab worker could have been working with samples of Yersinia pestis to make improved vaccines for regions in the world where the plague is endemic, Wren noted, adding that "if Yersinia pestis was weaponised, a vaccine for military personnel may be desirable."

Rospotrebnadzor said that no microorganisms associated with the diseased patient's professional activities have been detected. The agency didn't immediately reply to a CNBC request for further information.

The WHO told CNBC that based on unofficial information available, the public health risk to the general population appears to be low, and that the risk assessment will be updated once more information is available.

— CNBC's Jenny Lee contributed to this report