It's my understanding that the vaccine doesn't keep a person from catching or spreading the virus
"Catching" is a complicated term, spreading somewhat less so but fraught with complications.
The technical phrase for "can't catch it at all" would be "sterilizing immunity" which would prevent any replication before the immune system kicked in. And... yes that is not probably generally conferred... except maybe in some small percentage... for probably a pretty short period of time..
However, all of the vaccines that are generally available in the western world (<- this phrase was carefully worded) appear to provide pretty decent "effective immunity" though, which kicks in the immune system fast enough to stop replication before it has a chance to replicate enough to:
- have a high probability of spreading to other people
- have a high probability of causing significant symptoms or side effects
- have a high probability of replicating long enough to mutate sufficiently to bypass previous immune reactions (this is really important)
The astute observer will note that I'm talking about probabilities there, there is still
some chance of any of the above happening (back to the old saw about sufficiently large numbers of monkeys spontaneously producing Shakespear which I would personally consider thoroughly debunked by the evidence provided by the existence of the internet.. which is still an orders of magnitude smaller sample than even one short viral infection, but I digress.. the point is that with sufficiently large numbers of chances even highly unlikely things can and sometimes do happen, sometimes more frequently than you'd naively imagine).
So while there is still some large number of potentially infectious individuals (which seems distressingly likely to continue far longer than I would like..) some precautions that provide some additional reduction in the probability of events happening is sound public and, generally speaking for myself, personal policy.
I am mildly encouraged by the small number of unique mutations that have thus far occurred. Many of the mutations have occured repeatedly in different events and in slightly different ways, but there appear to be 3 or maybe 4 depending on how you count actually effective changes. This would generally bode well for follow on shots having relatively high efficacy for some timeframe.
The main risk is definitely that we still have large centers of infection, or we have large populations partially vaccinated that can provide breeding ground for further mutations that may be unique in some new way (the Brazilian P1 variant that seems to be both highly infectious, bypassing to some extent antibody reactions from previous variants, as well as well as having a strong effect on younger folks as an example of a concerning combination). So if we can reduce the probability of even mild infections, we're reducing the probability of more mutations, which reduces the probability of worse outcomes.
It's all a simple numbers game, basic statistics (which means it's also counter intuitive to about 90% of the population).
Edit: Both of us just got Moderna #2 about a week ago, sore arms, one day of feeling blechy. Nothing else..