The player, who was not publicly identified, traveled with the team to Utah for Saturday's 33-17 victory over the Utes and had tested negative within 36 hours of travel and again on game day, according to a USC statement.
The player tested positive for COVID-19 on Monday and the university was informed Monday night. He was not present in the facilities or at practice on Sunday or Monday, the statement said.
The player is symptomatic and has been quarantined. USC Student Health, Utah Athletics and the Pac-12 Conference have been notified.
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There were no other positive tests among the team Monday.
The Trojans (3-0) are scheduled to play host to Colorado (2-0) on Saturday at the Los Angeles Memorial Coliseum.
USC issued its weekly report on coronavirus testing of its athletes on Monday with one member of a team not actively participating in workouts on campus testing positive among the 1,157 test since the last report Nov. 16.
Since testing began in mid-June, there have been 26 positive tests among the 10,048 conducted on USC athletes. The percentage of positive tests is 0.26%.
According to USC, the school conducts antigen testing for all football players, coaches and support staff in close contact with players Monday-Friday. The testing occurs in the morning to ensure that everyone is cleared with a negative test before participating in practice.