Initial preliminary results from the Canadian Partnership for Tomorrow’s Health (CanPath) COVID-19 Antibody Study, based on close to 6,000 dried blood spot samples collected between February 8 and May 17, 2021, show a high degree of variability in the level of antibodies produced by a single dose of a COVID-19 vaccine. These findings highlight the importance of accelerating second doses as the Delta variant continues to spread, particularly with the vast majority of Canadians having received only a single vaccine dose.