Pfizer is set to ask U.S. regulators to allow emergency use of its COVID-19 vaccine, starting a process that could bring first shots as early as next month. Friday’s step comes days after Pfizer and its German partner BioNTech announced its vaccine appears 95% protective in a large but not yet finished study. Over the next few weeks, the Food and Drug Administration and its scientific advisers will have to decide if there’s really enough evidence to allow emergency vaccinations. If so, first supplies will be scarce and rationed. Experts warn it likely will be spring before there’s enough for everyone.