coronavirus vaccines continue to roll out to vulnerable populations across Canada, health officials are looking at data about the effectiveness of a single dose in preventing COVID-19 illness.A new letter by two Canadian experts published in the New England Journal of Medicine (NEJM) last week stated that with a 92.6 per cent efficacy, the first dose of the Pfizer vaccine was “highly protective.” Delaying second dose of coronavirus vaccines is ‘risky gamble,’ experts say During a news conference on Feb.