Joanne Goerler didn't want to retire in January from her teaching job at the John M. Marshall Elementary School, where she had worked since 1989, but she didn't really have a choice. The Covid-19 pandemic posed a life-threatening risk to her immune system, which had been compromised by a blood cancer called myelodysplastic syndrome, a surprise diagnosis Ms. Goerler received in the spring of 2018 after having some routine bloodwork.