The speaker has stayed inside for a whole year, and the poem reads like a sealed-off diary entry. Staying home is presented as a kind of protection: plague and war are things to dodge, like an infected kiss. That comparison is odd and a little darkly funny, since a kiss is normally about closeness, and here closeness is the danger.
The tone shifts near the end. The speaker admits not even knowing what the weather is like, which shows how cut off they are from the ordinary world outside. The last lines turn to the heart, which might sink or might sing, and the speaker doesn't know which is more likely. The poem doesn't settle it.
The mood is quiet, uneasy and suspended. There's relief at being safe, but also loneliness and a sense of waiting. The missing punctuation and the way sentences run into each other help, because the lines drift along like days that blur together.