A Florida teacher who couldn't get vaccinated because of her cancer treatment died of COVID-19. Her union says she caught it from her classroom, which had no mask mandate.

• Florida elementary school teacher Kelly Peterson died of COVID-19 complications on Monday.
• She was advised not to get a COVID-19 vaccine by her doctor due to her leukemia treatment.
• Her sister and union believes Peterson got infected in the classroom, where masks were not mandatory.

A 41-year-old Florida teacher whose doctor had advised her against getting vaccinated has died of COVID-19 complications after she was forced to return to in-person teaching where there was no mask mandate, local outlets say.

Kelly Peterson was not vaccinated against the coronavirus because she had leukemia and her doctor advised against getting the shot in her already weakened state, her sister, Christin, told KTVU.

Lake Shipp Elementary School announced Peterson's death in a Facebook post on Monday, saying she "touched hundreds of students' lives" and "made a lasting impression on us all."

Both Peterson's sister and the Polk County teacher's union said that she contracted COVID-19 in the classroom, KTVU reported. However, it should be noted that it's almost impossible to know how someone contracted COVID-19.

Florida Gov. Ron DeSantis has banned mask mandates in the state, but several schools have defied the order.

Peterson's sister told KTVU of the doctor's warnings against the COVID-19 vaccination: "Because her leukemia was so bad at this point, their concern was by getting the vaccine that potentially could put too much stress on her body."

Cancer patients and survivors are encouraged to get the vaccine, but to discuss the decision with their doctor first, according to the American Cancer Society. ACS said the main question about vaccines and cancer is not whether the vaccine is safe, but whether it is as effective in people with already compromised immune systems.

Last year, Peterson worked remotely, but her sister said she was forced to return to in-person learning this school year, even though she's immunocompromised.

"With all the COVID cases this year and her medical situation, she should have been a virtual teacher this year. The school didn't offer that," Peterson's sister told The Ledger.

Both the Polk County School District superintendent and the Lake Shipp principal did not immediately respond to Insider's request for comment.

Lorinda Utter, one of Peterson's coworkers, told The Ledger that Peterson took every precaution in the classroom, wearing a mask at all times and sanitizing surfaces regularly.

"She did everything she could to try to stay away from COVID," Utter said.

Peterson was terrified of getting COVID-19, her sister said, and knew the effects it would have on her if she contracted it.

"She had voiced concerns many times that if she contracted COVID, she was afraid that it would kill her, and unfortunately that's what happened," Christin Peterson told KTVU.

Christin Peterson also said she hopes her sister's story will encourage more people to get vaccinated.

Stephanie Yocum, president of the Polk County Teachers union, said she hopes parents "set good examples" for their kids by wearing masks.

"If wearing a mask can keep somebody from dying, that should be something that every person should do right now," Yocum told KTVU.