The new C.D.C. guidance is the latest sign of an accelerating race for a vaccine to greatly ease a pandemic that has killed more than 184,000 Americans. The documents were sent out on the same day that President Trump told the nation in his speech to the Republican National Convention that a vaccine might arrive before the end of the year.

Over the past week, both Dr. Anthony S. Fauci, the country's top infectious disease expert, and Dr. Stephen Hahn, who heads the Food and Drug Administration, have said in interviews with news organizations that a vaccine could be available for certain groups before clinical trials have been completed, if the data is overwhelmingly positive.