The problem with this question is that there are likely to be multiple vaccines. Each will have its quirks. The first round of vaccines will require two doses

The problem with this question is that there are likely to be multiple vaccines. Each will have its quirks. The first round of vaccines will require two doses a few weeks apart. Each one of those requires a visit. Failure to get the second shot will likely lead to severely reduced efficacy. The first two vaccines likely to be available in the US will be mRNA vaccines. They are based on an entirely new platform that's never been deployed at scale. That's going to lead to some surprises - possibly good but maybe not so good. Later vaccines will be more traditional such as the adenovirus-based vaccine being developed by J&J. Numerically, we're likely to see severe reactions or even death in hundreds or thousands of cases at least. The clinical trials are being run on tens of thousands of patients only, and can only pick up so many reactions. As hundreds of millions get vaccinated, we'll see the full range of effects, some of which could be serious even with the safest vaccines. It's important to keep this in perspective as the news of incidents starts to come out. I sometimes hear the statement "I'm waiting until it's proven safe." If that's you, you might want to ask yourself what evidence you'll need to know the vaccine is safe.