I was thinking earlier that the young adult resistance/ambivalence is a huge problem with the goal of vaccine induced herd immunity. Whatever percent of the population is calculated to need immunity to reach herd immunity, the calculation is based on a roughly random sampling. If the number for COVID is 70% (it could be higher or lower, I don't know), the assumption is that the virus only has 30% of the population to spread to and it is unlikely that an infected person comes into contact in a way that they can transmit it with one of the 30% not immune.
However, with the vaccination rate being so varied by age, I don't think it will work because for most situations where spread is likely to occur, people are having close contact with others that are near their age. College students don't typically invite their grandparents to frat parties.
If you end up with 70% of the population vaccinated but if under 30 is only at 35%, there will not be herd immunity in the under 30 population who spends a lot of time with each other. Based on the trends to date, it will take a really successful outreach program to get the under 30 crowd anywhere near the required percentage vaccinated.
I'd say this is roughly accurate: To reach herd immunity, you need immunity spread pretty evenly through the herd. That means across societal groups, including geographic and age.
While it might be possible to reach herd immunity without vaccinating children (who already are less likely to transmit the disease), you aren't going to reach it in localities that only have 30% vaccination rates. It's going to be difficult to reach it if you have a workplace where only 30% of the employees are vaccinated.
I suspect we may see a false sense of security by late summer/fall. Cases will have declined massively due to both vaccines and seasonal effects. Everyone will think they are out of the woods. And I don't think we will see a massive surge like fall 2020, but I think you'll have some very significant local outbreaks in communities with low vaccine uptake. And large local outbreaks could spread to become small surges in places with decent vaccine uptake.