What kind of scientific evidence supports this change in quarantine length?

Individuals who have been exposed to COVID-19 can take up to 14 days to show symptoms. Modeling studies show that the risk of transmission after day 10, particularly if an individual has a negative PCR test at least 48 hours prior, is reduced. Additional information: https://www.cdc.gov/coronavirus/2019-ncov/more/scientific-brief-options-to-reduce-quarantine.html