It’s a dark time in the struggle with the coronavirus, particularly in the United States, where infections and hospitalizations have surged.

But amid the gloom comes some light: A trial by the drug maker Pfizer has returned preliminary results suggesting that its vaccine is 90 percent effective in preventing Covid-19.

With the virus raging, how strong is this new ray of hope?

Guest: Carl Zimmer,a science writer and author of the “Matter” column for The New York Times.

Background reading:

• Pfizer has announced positive early results from its coronavirus vaccine trial, cementing the lead in a frenzied global race that has unfolded at record-breaking speed.
• Meet the couple behind the German company, BioNTech, that partnered with Pfizer to develop the vaccine.