The flood of pandemic literature shows no sign of letting up. In the three-plus years since the COVID-19 lockdown, we have seen fiction from the likes of Gary Shteyngart, Elizabeth Strout and many others. Now Sigrid Nunez, author of “Sempre Susan: A Memoir of Susan Sontag” and the National Book Award-winning “The Friend,” has written a pandemic novel called “The Vulnerables.” The title refers to the groups of people, including the elderly, considered at high risk of getting severely ill at the start of the pandemic in spring 2020.