Novelty means severity
Dylan Morris has an essay in Zeynep Tufecki’s Insight newsletter about COVID-19 and how Novelty Means Severity is the key to the pandemic, because SARS-CoV-2 is a novel virus (one our immune systems haven’t seen before):
Unless we eradicate SARS-CoV-2—possible but not likely, especially in the short term—just about everyone is going to encounter the virus sooner or later. But those who have adaptive immunity from infection or vaccination may not get sick at all. Even if they do, they will be less likely to get very sick or die.