Variants
This preprint says that a Delta infection (by itself) does provide some protection against Omicron (in mice and test tubes), but an Omicron infection (by itself) does not provide good protection against Delta. However, if you have an Omicron or Delta infection plus a vaccine, then you get good protection against the other variant.
This pre-preprint found the same thing in test tubes. In fact, this one found that protection against Delta was better than protection against Omicron after Omicron infection. (This might be entirely because the vaccine is better against Delta than Omicron, I couldn’t tell from the paper.)
This Twitter thread says that the risk of Omicron while masked is lower than the risk from Delta when masked. Basically, Omicron is an upper respiratory disease while Delta includes the lower respiratory system, and the upper respiratory system makes bigger particles (as they showed in this preprint from October).
I have to wonder why so many people have gotten sick with Omicron. Have that many people gone without masks?
Pathology
This Twitter thread observes that Omicron seems to be less severe in adults but more severe in children:
Interestingly, the total pediatric hospital occupancy hasn’t risen:
Recommended Reading
This article talks about grieving for people who died of COVID-19.