What scientists are learning about how long Covid-19 immunity lasts

Covid-19 continues to confound us all, with a growing list of symptoms, unexpected modes of transmission, and a wide range of outcomes, from benign to severe.

More than 600,000 people around the world have been killed by the virus, as of July 22. And for many survivors, Covid-19 is becoming a long-term condition too, with symptoms lasting for months.

Yet the majority of people infected with the virus recover on their own. And without a vaccine or much in the way of treatment options, the human immune system — a...