Yet predicting the strength and duration of the immune system's response, whether to a live infection or a vaccine, is anything but straightforward.
In the fall of 2016, Columbia University researchers began periodically swabbing the nasal passages of 191 volunteers, analyzing the genetic material within for a variety of respiratory viruses.
During the next year and a half, 86 people became infected with coronaviruses—milder cousins of the one now causing so much havoc. Twelve tested positive for the same one at least twice, including three people who each were infected three times with a coronavirus nicknamed OC43.
