But based on past vaccines for other viruses and the way they and the covid vaccines help your body be better prepared to fight the virus, it's a reasonable guess that they may well at least limit spread, perhaps even close to prevent it.

So you're right to point out that it's not a sure thing, but I (admittedly, not an expert, but basing on views of experts) also think it would be more surprising if the vaccines didn't slow spread at all, the bigger question will be to what extent (both with and without current protective tools like masks and social distancing).