The World Health Organization (WHO) said on Saturday that the cause of pneumonia that killed an employee at a plague research facility in Siberia remains unknown. Russian authorities have reported no confirmed cases of plague connected to the incident.

The WHO said Russia’s health regulator reported that tests conducted on the deceased worker and people who had contact with her came back negative for the high-risk pathogens examined. Those contacts have not developed symptoms, and officials have found no additional cases of severe pneumonia linked to the death.

“Based on the information provided … (the) WHO has not identified evidence of ongoing transmission associated with this event. A key uncertainty remains ​the absence of a confirmed diagnosis in the deceased laboratory worker,” the organization said in a statement.

The 28-year-old laboratory employee, identified by Russian media as Darya Shipilova, became ill and died on October 2 under circumstances that have not been disclosed. Her death prompted authorities to quarantine approximately 200 people and sparked concerns about a possible outbreak on social media.

Health officials worldwide have remained particularly vigilant about unexplained fatal pneumonia cases since the COVID-19 pandemic began with a cluster of rapidly spreading respiratory illnesses in China. However, the precise cause of an individual pneumonia case is not always identifiable.

Russian officials have stated that the employee had received the appropriate vaccinations. They also said no microorganisms associated with her laboratory work were detected in her body.

On Thursday, Russian authorities announced that medical checks of everyone who had contact with the employee had been completed. Moscow also said it had shared all relevant information with the WHO in accordance with established procedures.