Local media identified the 28-year-old woman as Darya Shipilova, an employee of the Irkutsk Anti-Plague Research Institute of Siberia and the Far East.
Russia’s consumer watchdog, Rospotrebnadzor, said a special set of measures had been implemented following the case and that expanded testing of samples from the patient had found no microorganisms associated with her professional activities.
The agency described the illness as “pneumonia of unknown etiology,” without confirming that the patient had contracted plague.
The case has nevertheless prompted concern because of the woman’s work at an institute specializing in research on plague and other infectious diseases.
Alexei Tsydenov, the governor of the neighboring Buryatia region, said in a post on his Telegram channel on Saturday that a woman in Irkutsk had “died, maybe of the plague”.
Tsydenov sought to reassure residents by saying Russian authorities had confirmed that the woman had not traveled to Buryatia.
Meanwhile, media reports said authorities in Irkutsk had placed nearly 200 people under medical observation following the case.
The US Department of State said Washington was “monitoring the situation closely with the CDC (Centers for Disease Control and Prevention) and our other interagency partners”.
“We encourage Russian authorities to share accurate information quickly and openly,” the State Department official said.
Pneumonic plague is caused by the bacterium Yersinia pestis, which is usually found in small mammals and their fleas.
According to the World Health Organization, plague is highly infectious and can be fatal if it is not treated at an early stage.
Russian authorities, however, have not confirmed that the woman’s death was caused by plague, while initial expanded testing reported by Rospotrebnadzor found no microorganisms linked to her professional activities.