LOUISVILLE, Ky. — Cadell Walker rushed to get her 9-year-old daughter Solome vaccinated against COVID-19 — not just to protect her but to help stop the coronavirus from spreading and spawning even more dangerous variants. Scientists agree. Each infection — whether in an adult in Yemen or a kid in Kentucky — gives the virus another opportunity to mutate. Protecting a new, large chunk of the population anywhere in the world limits those opportunities. Vaccinating kids also means reducing silent spread, since most have no or mild symptoms when they contract the virus. When the virus spreads unseen, scientists say,...