A quick analysis of Texas schools to illustrate a couple points about symptomatic covid testing and contact tracing data:

(1) Many cases never get traced

(2) Many infections don't become "cases" because they aren't tested or reported (1/3 reported is probably too generous)

Data is July 27 2020-Jan 17 2021. I only included schools that appeared to be doing any investigation about transmission source (excluded those with 100% unknown) https://dshs.texas.gov/coronavirus/schools/texas-education-agency/

This doesn't just apply to schools! Case data in general captures a minority of infections. And industry groups, like restaurant associations, use low contact tracing numbers to argue that there is little risk