Actually, that's not true. 5% of people getting either vaccine still developed symptomatic infections (not severe, except for 1 case) and the degree to which vaccinated people might become infected but asymptomatic and potentially infectious was not measured in either clinical trial. However, the Moderna trial did look at asymptomatic infections in the nose after the first shot and saw reductions vs. placebo, but the results are merely suggestive and not conclusive (not enough statistical power); Pfizer did not explore this, but given how similar results have been for both vaccines, one would expect them to share this element of performance.
But common sense says that a primed immune system ought to be able to prevent infections completely in more people than those who aren't vaccinated but get infected. That's why many think we'll still need distancing/masking for some time, but IMO, we're going to see case rates start to plummet by the end of January, after vaccinating most health care workers, LTC residents and a good chunk of front line workers and the elderly in general (plus we'll likely be near 30-35% of the general population having been infected and no longer infectious). If we see cases plummet by February, perhaps restrictions will be loosened for some events like sports, especially if one can show a record of vaccination. I hope so, as I really want to go to the NCAAs this year, given what happened last year.