The two leading vaccines right now (Pfizer and Moderna) require two shots each, so you'd need to factor in timing of the second shot (a month?). You'd also have to factor in the time required to develop resistance (a couple months?). It's also possible that people that had COVID a year earlier may have the resistance dropping off (just guessing, no data that I'm aware of). I also wouldn't be surprised to see some hiccups in the supply and distribution of the vaccine. Not pointing fingers at anyone, it's just that that sort of things happen in supply chains. Early vaccines may be efficient when you can hit up large populations in nursing homes or hospital staff. But maybe it's less efficient if folks have to wait in line for 1-2 hours at CVS.

Also, different parts of the population have different willingness to get the vaccine. For the sake of TOS, we'll say that the American League cities/states all want the vaccine, but the National Leagues cities are resistant. So there's be the risk of some outbreaks in National League cities until they can be convinced that the designated hitter, I mean the vaccine, is good for baseball in general.

The "experts" I've seen have said that there's a good chance that we'll be in good shape for football in the Fall of 2021, but maybe not baseball in the spring of 2021.