The Trump administration has selected five companies as the most likely candidates to produce a vaccine for the coronavirus, senior officials said, a critical step in the White House’s effort to deliver on its promise of being able to start widespread inoculation of Americans by the end of the year. Noah Weiland and David E. Sanger write in the New York Times that the five companies are Moderna, a Massachusetts-based biotechnology firm, which Dr. Fauci said he expected would enter into the final phase of clinical trials next month; the combination of Oxford University and AstraZeneca, on a similar schedule; and three large pharmaceutical companies: Johnson & Johnson, Merck and Pfizer. Each is taking a somewhat different approach.