A mysterious death at a Russian research lab has sent health authorities scrambling and Wall Street into a familiar, headline-driven tailspin.

After jumping nearly 7% in the previous session, pandemic winner Moderna snapped its two-day winning streak on Tuesday as shares sank 7.8%. Novavax fared even worse, tumbling 9.9% to record its sharpest single-day loss since August of last year.

The catalyst: a sudden, unexplained illness inside a research facility. Local media identified the victim as 28-year-old Darya Shipilova, a lab technician at the Irkutsk Anti-Plague Research Institute of Siberia and the Far East. She died earlier this month, just two days after falling ill at work.

Russia’s national public health agency, Rospotrebnadzor, has pushed back on allegations of a lab leak or plague infection. “No microorganisms associated with the employee’s professional duties were detected in samples taken from the patient,” the agency said in a statement.

Although some reports and a social media post from politician Alexey Tsydenov linked Shipilova’s death to pneumonic plague, Rospotrebnadzor stated she died from “pneumonia of unknown etiology.” The phrasing echoes language used at the onset of the Covid-19 outbreak in Wuhan, China, in late 2019, describing a condition whose underlying cause or origin cannot be medically determined.

Pneumonia is a general lung infection caused by various bacteria, viruses, or fungi. In contrast, pneumonic plague is a rarer, more severe infection caused specifically by the bacterium Yersinia pestis. It is nearly always fatal without prompt treatment.

While standard community-acquired pneumonia typically responds to oral antibiotics like amoxicillin or azithromycin, pneumonic plague requires immediate, aggressive treatment with longer-term intravenous or intramuscular antibiotics.

International media have widely circulated claims of pneumonic plague, despite those remaining heavily disputed. Tsydenov later walked back his statement, editing his Telegram post to clarify that Shipilova had only “possibly” died from the disease.

Local media coverage has done little to clarify how she contracted the illness. One account claimed that it happened while Shipilova was on vacation in Thailand and that she was admitted to the hospital after returning to Russia. Another suggested a test tube containing the bacterium shattered, though Rospotrebnadzor denied that any such accident took place.

Regardless of the underlying cause, Shipilova’s death triggered an extensive monitoring effort in the city of Irkutsk. As of Oct. 2, at least 189 people were under medical observation, while a handful of local hospitals had imposed quarantines.

Officials outside Russia have dismissed concerns that the illness will spread. The U.S. State Department and the Centers for Disease Control and Prevention said Sunday that they were monitoring the situation, adding that the safety and security of U.S. citizens is of the “highest priority.”

Meanwhile, the World Health Organization confirmed Monday that it is coordinating with Russian authorities to verify the outbreak and assess potential international risks. Based on available information, it considers the immediate threat to the general public low, although the agency has pledged to update its assessment as more details emerge.

The sharp pullback in Moderna and Novavax stocks came Tuesday after the WHO downplayed the transmission risk. The move followed a speculative rally a day earlier.

While both companies make vaccines against viral pathogens that can cause viral pneumonia, neither produces vaccines for the leading cause of bacterial pneumonia, nor do they offer acute treatments for active infections.

Ultimately, the reaction underscores how quickly the pandemic trade can resurface even when conditions fall short of a global crisis. The volatility mirrors earlier outbreak-driven panics like May’s hantavirus scare, proving just how sensitive markets remain to headlines.

The WHO defines a pandemic as the worldwide spread of a new disease across multiple continents. With the current threat level nowhere near that threshold, the sudden swing in vaccine-linked stocks reflects market jitters rather than fundamentals.