edit2: actually, I see the denominator there is total population not cases but I still don't follow.

There have been 19,850,744 cases in 18-49 year olds [0] and 63,207 hospitalizations [1] which suggests at 0.3% infection hospitalization rate..

[0] https://covid.cdc.gov/covid-data-tracker/#demographics

[1] https://gis.cdc.gov/grasp/COVIDNet/COVID19_5.html#virusTypeD...