How it works: The more widely a virus can spread, the more opportunities it has to mutate. If the U.S. and ultimately the world don’t vaccinate a sufficient percentage of the population, we’ll be setting ourselves up to let the virus keep spreading, and keep mutating, continuing to give us new variants that will continue to pose new threats... But if we don’t control the virus well enough, then even years into the future, we could be living through more new variants — some of which might be more deadly, some of which might be more resistant to vaccines, some of which might be more dangerous for certain specific populations. That would translate into an ongoing risk of illness or potentially death for unvaccinated people and new races to reformulate vaccines as new variants keep emerging.