Canada’s vaccination advisors aren’t walking back a controversial “first-dose-first” COVID vaccine strategy of spacing shots up to four months apart, arguing it’s critical to get at least one dose into more bodies as quickly as possible. Last month, the National Advisory Committee on Immunization, or NACI, endorsed stretching the interval between doses up to 16 weeks — a gap longer than any endorsed by any other country — to get a greater proportion of the population at least partially vaccinated. The officially authorized window is 21 days between doses for Pfizer-BioNTech’s vaccine, and 28 days between Moderna shots. On Wednesday, the panel stood by its earlier “rapid response” recommendation, saying delaying the second shot will bring faster population-level protection. “Informed, expert opinion based on principles of immunology indicate that a longer interval between priming and boosting doses of a vaccine series results in a better, more durable response,” NACI said.