Pfizer Inc’s COVID-19 vaccine could be delayed in order to cover all priority groups as the first one is highly protective, two Canada-based researchers said in a letter published in the New England Journal of Medicine.The vaccine had an efficacy of 92.6 per cent after the first dose, Danuta Skowronski and Gaston De Serres said, based on an analysis of the documents submitted by the drugmaker to the U.S.