Scientists have found clues that the world’s leading COVID-19 vaccines offer lasting protection that could diminish the need for frequent booster shots, but they caution that more research is needed and that virus mutations are still a wild card. Critical studies are underway, and evidence is mounting that immunity from the mRNA vaccines made by Pfizer-BioNTech and Moderna does not depend exclusively on antibodies that dwindle over time. The body has overlapping layers of protection that offer backup. Pfizer and Moderna have fueled booster questions by estimating that people might need yearly shots, just like with flu vaccinations, and the companies are working to have some candidates ready this fall. But they will not decide when boosters get used — that will be up to health authorities in each country. Other experts say boosters may be needed only every few years.