According to a statement by White House press secretary Jen Psaki:
On Monday morning, a mid-level staff member, who does not regularly have contact with the President, received a positive result for a COVID-19 test. Three days earlier, on Friday, that staff member had spent approximately 30 minutes in proximity to the President on Air Force One, on the way from Orange, South Carolina to Philadelphia, Pennsylvania. This staff member is fully vaccinated and boosted, and tested negative prior to boarding Air Force One, as is required for everyone traveling with the President. This staff member did not begin to experience symptoms until Sunday, and was tested on Monday . . .
This morning, after being notified of the staffer’s positive test, the President received a PCR test and tested negative. He will be tested again on Wednesday.
As CDC guidance does not require fully vaccinated people to quarantine after an exposure, the President will continue with his daily schedule.