The World Health Organization has (as of January 2022) identified several variants of SARS-CoV-2, 5 of which are designated as variants of concern. This designation indicates that the variant is more serious than previous variants of the virus, in that it is more transmissible, or evades current vaccinations or treatments. According to the WHO, 4 of the variants of concern emerged before vaccinations were available or widely deployed. Around 25-28% of South Africans had recieved at least 1 dose of a COVID vaccine at the time the omicron variant was identified there, while the global average vaccination share was around 50%.
Dr. William Schaffner from the Division of Infectious Diseases at Vanderbilt University Medical Center said to CNN in July 2021 "Unvaccinated people are potential variant factories [...] The more unvaccinated people there are, the more opportunities for the virus to multiply [...] When it does, it mutates, and it could throw off a variant mutation that is even more serious down the road."
Dr. Amesh Adalja, from the Johns Hopkins Center for Health Security said to Poynter in August 2021, "The delta variant emerged in a largely unvaccinated country [...] The more the virus spreads, the more chance there is for variants to emerge."