Covid-19 vaccines have begun rolling out to health workers like Julio Bercian, a CNA in Massachusetts. Ending the pandemic will require vaccinating millions more people. | Jessica Rinaldi/Boston Globe via Getty Images

Covid-19 vaccines are here, but key questions remain.

Two highly effective Covid-19 vaccines are now being administered across the United States, and more are in the pipeline. Almost 2 million people have already received the first of two doses of these vaccines, and officials are aiming to immunize one-third of the US population by the end of March 2021.

It’s a stunning accomplishment for a disease that was barely known to the world a year ago, and it means that an end to the crisis is in sight. Yet the US remains in the worst throes of the pandemic to date, with hospitalizations and deaths continuing to break records.