Variants are inevitable with any virus and some of them will be worrying because they either spread faster or the current vaccines may not work as well on them. The way to prevent even more variants is to limit its spread by vaccination. As it said in the fact check article:
Joe Grove, a virologist at the Institute of Immunity and Transplantation at University College London, explained that even though vaccination could drive some virus changes, “it is not credible that widespread immunity will drive the emergence of a monster virus.”
Geert pointed out that the 1918 flu pandemic had few strains and was over in a year, with no vaccination. He also said the problem with mass vaccination is that however fast it's done, there is still a time lag while the body is developing specific antibodies, which is enough time for mutants to become predominant.
He doesn't say anything about a monster virus during the interview (almost 2 hours).
From what I can understand, he is putting a lot of emphasis on the importance of the innate (unspecific) immune system and worrying that this could in future, when you have new variants, be rendered ineffective by the acquired antibodies to the vaccines.
