CHICAGO/LONDON : Scientists on Monday said initial trial results for Pfizer Inc and BioNTech's coronavirus vaccine far outpaced their expectations for protection against a completely new disease, but that many questions remain unanswered.The drugmakers said their vaccine was more than 90% effective at preventing Covid-19, based on data from the first 94 people in the trial to become infected with the coronavirus.The efficacy rate means that the overwhelming majority of infections occurred among people who received a placebo rather than the vaccine.The 44,000-volunteer study was initially designed for a first interim analysis of whether the vaccine was working after 32 participants developed Covid-19.Dr.