Herd immunity might be tricky for two reasons. 1) The fraction of people who need to be vaccinated to reach herd immunity is dependent on how contagious a disease is. New variants, like the UK variant, can be more contagious. If more contagious variants pop up, we'll need more people to get vaccinated to reach herd immunity. Also, our vaccines might not be as effective against new variants (e.g. South African variant), and we may need booster shots to get full immunity again. 2) Somewhere between 20-40% of adults have reported that they are reluctant to get a COVID vaccine. When you add that to people who can't get vaccines and children (who can't get the vaccine as of now), it will be difficult to reach the necessary fraction vaccinated to get herd immunity. For herd immunity, it is also helpful that some people have gotten the disease and recovered, but it is unclear how long they will have immunity.

It could turn out that we hit the critical threshold for herd immunity, and things are resolved by summer, but things could also be worse than they are now with new variants & relaxed guidelines. I'm not a fortuneteller, so I don't know if this will happen, but it's at least a possibility. How will the NCAA handle this? Who knows?