In September 2018, a 26-year-old laboratory worker was offered a new job that would involve working with the vaccinia virus (VACV) – a large, complex virus related to smallpox.

She was told of the risks involved in working with VACV and given the option of receiving a vaccination against it, but ultimately declined, citing concerns about potential adverse effects of the vaccination and problems managing the infectious lesion at the site of the vaccination in her decision not to vaccinate.

Cut to her starting her job in December, where she accidentally pricked herself with a needle containing a potentially genetically altered strain of VACV after attempting to inject it into the tail of a mouse. She immediately washed the wound and notified her supervisor of the incident, who recommended she visit the local emergency department, the Centers for Disease Control and Preventions (CDC) details in its Morbidity and Mortality Weekly Report.

After being evaluated by two physicians, she was not advised to observe contact precautions in order to prevent transmitting the virus to others. However, she was monitored over the coming weeks and months, as the infection became worse.

Ten days after she pricked herself, her physician referred her to the CDC, who recommended closer monitoring of the infection. On day 12, she was taken to the emergency department with fever, swollen lymph nodes, pain, and a worsening of the infection in her finger.

She was given vaccinia antibodies to help her immune system fight off the virus, as well as antibiotics for a possible secondary infection in her open wound. Her fever and pain went down over the next 48 hours, though her infection didn't clear up until day 94.

The vaccinia virus was the live vaccine used to eradicate smallpox, an infectious disease that killed 300 million people in the 20th century alone, and is still the only infectious disease to be completely wiped out around the world. It's easy to genetically modify, and so is still used in research today. However that doesn't mean it's safe, and the CDC recommends that laboratories ensure their workers who come into contact with it know the risks, though vaccination itself is not mandatory.

What's especially concerning in this case is that it's not clear what strain the lab worker was infected with.

"Neither the patient nor the occupational health physician could specify the concentration or strain of VACV preparation used by the patient," the authors write in the case report.

"Upon inquiry, the study sponsor informed investigators that one of two genetically altered Western Reserve strains could have been involved. The patient was injecting multiple groups of mice with different strains and did not recall which strain she used when the needlestick injury occurred."

The injury eventually cleared up, though she was excluded from lab work for four months because of her necrosis and potential risk of VACV transmissions. Fortunately, despite not being told to follow precautions for the first 10 days of her infection, no others were infected.