The COVID Tracking Project at The Atlantic contains this summary statement:
Less than 1% of America’s population lives in long term care facilities, but, as of March 4, 2021, this tiny fraction of the country accounts for 34% of US COVID 19 deaths.
The authors of the COVID Tracking Project actually note that the correct percentage is higher:
CTP’s aggregated cumulative data for these states under-reports actual cumulative totals. This information can be found in our Aggregate Dataset. States vary in their reported cumulative data start date. Not all states and territories report long-term-care data.