Viruses with an advantage will outcompete other viruses, and will eventually make up the majority of virus particles infecting someone. If that infected person passes the virus to someone else, they'll be passing along the mutant version. If a mutant version is successful enough, it becomes a variant. But it has to replicate to do that. An unvaccinated person provides that opportunity. "As mutations come up in viruses, the ones that persist are the ones that make it easier for the virus to spread in the population," Andrew Pekosz, a microbiologist and immunologist at the Johns Hopkins Bloomberg School of Public Health, told CNN. "Every time the viruses changes, that gives the virus a different platform to add more mutations. Now we have viruses that spread more efficiently."