The requirement for a top-up isn't like an on-off switch. The vaccine isn't 100% effective at any point. The proportion of people who become susceptible again over time, and the proportion of them who become seriously ill are not really known even in relation to the current strains. To know more about that we will really need to see quite a lot of people catch it again.
Current tests suggests that the proportion of people who are functionally immunity drops reasonably quickly, so if we needed to keep people at that level we'd probably need to do everybody every year. It's conceivable but highly unlikely that we might need to do every six months. If we just need to vastly reduce the number of people getting very sick, we may be able to just vaccinate the over 50s, and just every couple of years. The decisions there aren't really medical, they're about working out how many people are dying or getting sick, and whether that is too many.
The new strains are a confounding factor. It's possible that the new strains aren't sufficiently good at by-passing the vaccines to make much of a difference. It's also possible in a better-vaccinated world there would be far fewer significant strains developing in a year, and in fact they wouldn't be a significant factor in a long-term vaccination strategy. However it's also possible that the vaccine needs to be modified every year like the flu vaccine to keep it effective. Obviously if this turns out to be the case, then more people will need vaccinating more often
Even in the worst case, doing 100m vaccinations a year isn't beyond us as a nation, and is better and more affordable than pretty much any other intervention on the table.
