This is what Reactive means:

Reactive test result: Indicates that your initial antibody test was reactive for COVID-19 antibodies. The Red Cross uses two antibody tests to generate our results. While it is uncommon, some donations may not complete both tests due to processing issues. The initial test detects antibodies to the spike protein of the virus, that are produced in response to viral infection or the vaccine. The other detects antibodies to a different part of the virus called the nucleocapsid protein, which are produced in response to infection, but not by the current vaccines. If a donor has had the COVID-19 vaccine, they will generate antibodies against the spike protein but not the nucleocapsid protein, which will only occur in the event of a COVID-19 infection. A reactive result on the antibody test usually requires about one to three weeks from the receipt of the first dose of a vaccine.