Dose No. 2 is more likely to pack a punch—in large part because the effects of the second shot build iteratively on the first. My husband, who’s a neurologist at Yale New Haven Hospital, is one of many who had a worse experience with his second shot than his first.

But much like any other learning process, in this one repetition is key. When hit with the second injection, the immune system recognizes the onslaught, and starts to take it even more seriously. The body’s encore act, uncomfortable though it might be, is evidence that the immune system is solidifying its defenses against the virus.

“By the second vaccine, it’s already amped up and ready to go,” Jasmine Marcelin, an infectious-disease physician at the University of Nebraska Medical Center, told me. Fortunately, side effects resolve quickly, whereas COVID-19 can bring on debilitating, months-long symptoms and has killed more than 2 million people.