Short answer: it's very encouraging news, but it's also very preliminary.
Pfizer's messenger RNA-based COVID-19 vaccine was always scheduled for interim analyses after prespecified numbers of COVID-19 infections an study volunteers. The plan had been to perform interim efficacy analyses analyses after 32, 62, 92, 120, and 164 cases of COVID-19 in the study population, including both vaccine and placebo recipients. Today's announcement was at the 92 case count, though it actually turned out to be 94 cases. The protocol had set success criteria for this stage at 62.7% efficacy; they also set futility criteria, if efficacy was 38.6% or less it could indicate that it was futile to continue the study since they would be unlikely to reach 50% efficacy after final evaluation.
As an aside, note that it's a pretty complicated issue to perform interim analyses in blinded randomized controlled trials. These "peeks" at the data essentially violate some of the assumptions of standard statistical testing and require correction factors. The statisticians I hang out with like to call this "spending alpha:" each time you look at the data you must correct the statistical analysis, effectively lowering the p value making it tougher to achieve statistical significance. Also note that the investigators and trial volunteers are still blinded as to whether placebo or vaccine was given to an individual. It is a separate data monitoring committee that is actually unblinded for these peeks to see what the infection rates are in vaccine and placebo groups.
So, we'll need to see if this early hint of high efficacy is actually sustained to the end of the study. If so, this will be terrific. Also, we'll eventually know more about the range of disease severity in volunteers as well as some indication of how the vaccine performs in different age groups.
This is also good news for the Moderna COVID-19 vaccine because it uses a similar messenger RNA construct for its vaccine.
My division chief alerted me to a new, cool NY Times interactive web site for COVID vaccines, check it out if you get a chance.