AstraZeneca (AZN 0.28%) signed a large coronavirus vaccine-delivery deal with Canada on Friday. The big pharmaceutical company will deliver up to 20 million doses of its coronavirus vaccine candidate, AZD1222.

This isn't the first time AstraZeneca has signed a commitment to deliver enormous quantities of its coronavirus vaccine candidate before it's had a chance to prove itself safe and effective in a phase 3 clinical trial. In June, the company agreed to supply Europe with up to 400 million doses of AZD1222 with deliveries beginning by the end of the year. The company also has a deal with Brazil to provide around 30 million finished doses of the vaccine by December.

Masks and a COVID-19 vaccine.

Image source: Getty Images.

Recently, clinical trials with AZD1222 were paused around the globe after a clinical trial participant in the U.K. experienced an adverse event serious enough to require hospitalization. Following inspections from independent data monitors, phase 3 studies around the globe have been allowed to continue, with the exception of one key location. To date, clinical trials with AZD1222 in the U.S. are still paused.

AZD1222 employs the hollowed-out husk of a different virus to deliver blueprints for portions of SARS-CoV-2, the virus responsible for COVID-19. The viral vector AZD1222 uses can't replicate itself, but there's always a slight chance a significant percentage of the population will have immune systems that overreact to a delivery vehicle that never elicits reactions otherwise

At the moment, there are nine SARS-CoV-2 vaccine candidates in clinical trials, but AZD1222 is arguably in the lead despite its recent hold-up. In late June, AZD1222 became the first experimental vaccine program to begin a phase 3 trial, sponsored by concerned entrepreneurs in Brazil.