The best way to think about total severity in the U.S. is to imagine four concentric circles, where the risk of illness increases as you fan outward.

In the center ring are boosted non-seniors. This group seems quite safe from severe illness and well protected against infection. One ring out are doubly vaccinated Americans under 65 without immune-system complications. This group is more at risk for breakthrough Omicron cases—and could spread Omicron cases to more vulnerable people—but seems fairly protected against severe illness thanks to T-cell immunity. (As for these groups’ long-COVID risk from Omicron, I don’t think we have any good information yet. But previous strains have been associated with months of labored breathing and impaired lung function.)

In the third ring are people who are vaccinated, or even boosted, but still at-risk. This group, which includes seniors and immune-compromised individuals, deserves special care. People in the center-most circles should be cautious about mixing households with seniors, because in the case of a breakthrough, we can’t be sure that older people’s T-cell protection will protect them against severe illness. Remember that, among boosted seniors, age still matters: According to the CDC, people older than 85 have a COVID fatality risk that is roughly five times higher than the typical 65-year-old.

Finally, the fourth circle represents the broader unvaccinated population, both in the U.S. and throughout the world. Omicron seems so contagious that, even with an overall milder course of disease, we should expect a surge of hospitalizations among the unvaccinated.