“I thought to myself, ‘Should I mention to them that I had it?’” she said of her fellow passengers on her mostly empty flight, all of whom were talking about Covid-19. “Ultimately I chickened out.”

For Ms. Schneider, who lives alone and had isolated herself for two weeks after her early-March test, the time with her parents, playing board games and cooking familiar recipes, was restorative.

The flight she was supposed to take back to Seattle, however, was canceled because of a lack of passengers.

And on the flight she was able to find, there were so few fliers that she would have had no opportunity to speak to anyone even if she had wanted to.

“Empty,” she texted.

A Doctor Returns to the E.R.

It was midway through Dr. Dara Kass’s first 12-hour shift back in the emergency room on a recent Sunday that she received the most definitive news one can currently get about immunity to the novel coronavirus.

A few days before, Dr. Kass, who tested positive for the virus in March after treating Covid-19 patients at Columbia University Irving Medical Center, had given a blood sample to a study that she hoped would help others with the disease.

The study, conducted by researchers at Mount Sinai Hospital, aims to use virus-fighting antibodies from donors who have recovered to treat patients who are currently sick. To determine eligibility, the volunteers’ blood is screened for the right antibodies. Those who have plenty are sent to a blood center to donate plasma in an hourlong process that can yield enough to treat three patients.