Canada just made a major change to its coronavirus vaccine rollout

Of all the coronavirus vaccine candidates that cleared clinical trials with proven efficacy and safety records, the Johnson & Johnson drug is the only one that uses a single-dose regimen. The Pfizer/BioNTech, Moderna, and AstraZeneca/Oxford vaccines, currently in use in various countries, require two doses three or four weeks apart. The Novavax drug, which is yet to receive emergency use authorization, also uses a two-dose regimen. After nearly two months of COVID-19 vaccinations, researchers ha...