How far are we from a vaccine?

AZD1222’s efficacy (its ability to reduce the occurrence of symptomatic Covid-19 in those vaccinated) varies depending on the combination of doses administered during the ongoing trials in the UK and Brazil. According to the interim analysis, AZD1222 was found to bring down by 90% the number of symptomatic Covid-19 cases in those receiving a combination of half a dose of the vaccine followed by a full dose a month apart. But the efficacy seemed to drop to 62% when the vaccine was given as two full doses at least a month apart. It is significant that a combination of a lower dose and a higher dose were found to actually bring about a higher efficacy. This means that more people can be effectively vaccinated using the same limited supply, which might make the candidate attractive for governments targeting a larger population.