Dr. Caroline Quach-Thanh, the head of Canada’s National Advisory Committee on Immunization (NACI), has identified some of the biggest challenges around mass vaccination. These include identifying populations the most at-risk (including seniors, front-line workers and others at risk of contracting or transmitting the disease), and coordinating distribution in a manner that does not involve large gatherings that could help to spread the disease. Lack of a national immunization registry to track who has received the vaccine is also a concern as at least two of the vaccines require two doses.