The U. S. approved a new COVID-19 vaccine made by Moderna late Friday but with limits on who can use it — not a replacement for the company’s existing shot, but a second option. The new vaccine, mNexspike, is a step toward next-generation coronavirus vaccines. It's made in a way that allows for a lower dose — a fifth of the dose of its current COVID-19 vaccine, Spikevax — by refining its immune target. The approval “adds an important new tool to help protect people at high risk of in...