Dozens of Americans are rolling up their sleeves for a third dose of COVID-19 vaccine -- this time, shots tweaked to guard against a worrisome mutated version of the virus. Make no mistake: The vaccines currently being rolled out across the U.S. offer strong protection. But new studies of experimental updates to the Moderna and Pfizer vaccines mark a critical first step toward an alternative if the virus eventually outsmarts today's shots. Viruses constantly evolve, and the world is in a race to vaccinate millions and tamp down the coronavirus before even more mutants emerge. Already an easier-to-spread version found in Britain just months ago has become the most common variant now circulating in the United States, one that's fortunately vaccine-preventable. A different variant that first emerged in South Africa is causing more concern, after studies suggested that first-generation vaccines may offer less protection against that particular mutant. The study, run by the National Institutes of Health, isn't just testing Moderna's experimental variant vaccine as a third-shot immune booster. Researchers at Emory and three other medical centers around the country also are enrolling volunteers who haven't yet received any kind of COVID-19 vaccination.