Coronavirus deaths and cases in the U.S. have dropped markedly over the past couple of weeks but are still running at alarmingly high levels. And the effort to snuff out COVID-19 is becoming an ever more urgent race between the vaccine and the mutating virus. Deaths are running at an average of just under 3,100 a day, down from more than 3,350 less than two weeks ago. New cases are averaging about 170,000 a day, after peaking at around 250,000. The country’s top infectious-disease expert, Dr. Anthony Fauci, says the improvements appear to be the result of a natural plateau after the holiday surge — not the effect of the vaccine. And he urges continued vigilance.