Currently more people have COVID than at any point in the past [1]. This means COVID is evolving faster, as it has more chances to mutate. The danger of a new vaccine evading strain emerging is increasing along with the number of global infections. There's a risk that the rate of mutation accelerates to the point where we can't get updated vaccines into people fast enough.
It's sobering that only 4% of the world has been fully vaccinated in 4 months [2] and the job will get more difficult once all the easy to access people are done.
[1] Assuming cases in a 2 week period is a rough proxy for number of currently infected people. https://ourworldindata.org/explorers/coronavirus-data-explor...
[2] https://ourworldindata.org/explorers/coronavirus-data-explor...