question: As the COVID-19 pandemic continues to ravage the globe, many nations have started to relax stringent restrictions in an effort to restart the economy. While Member States of the European Union have approached reopening without the use of antibody testing for COVID-19, such testing may be central to a long-term, sustainable strategy for international travel, employment, and the allocation and monitoring of vaccines. As the use of antibody testing to dictate the enjoyment of individual freedom remains highly controversial, we describe its use in the context of three case studies (return to the workplace, travel and vaccination), applying the substantive legal balancing entailed in the proportionality principle. Differential treatment of individuals based on COVID-19 antibody test results can be justified through the proportionality principle, which offers a sound dividing line between a reasonable and responsible response and an unjust and discriminatory response.