It's worth noting that the first shot, by itself, does provide some measure of protection. I've seen estimates of around 50% effectiveness at preventing Covid. Certainly far better than no protection at all. Also, the timing of the second dose does not have to be exact. While the goal is to get it as soon as you hit the time frame (3 weeks for Pfizer, 4 weeks for Moderna), getting it a few weeks after that point does not change the effectiveness. So if you get the first dose and then there's a delay in getting the second dose, it's not the end of the world.