The global response to the microbe was predicated on the assumption that nobody would be immune to the “novel” virus. Like so much else that we were told, this was nonsense. From the BMJ in September (emphasis added):
It seemed a truth universally acknowledged that the human population had no pre-existing immunity to SARS-CoV-2, but is that actually the case? Peter Doshi explores the emerging research on immunological responses. […]
At least six studies have reported T cell reactivity against SARS-CoV-2 in 20% to 50% of people with no known exposure to the virus. […]
Another group led by Sunetra Gupta at the University of Oxford has arrived at similar conclusions of lower herd immunity thresholds by considering the issue of pre-existing immunity in the population. When a population has people with pre-existing immunity, as the T cell studies may be indicating is the case, the herd immunity threshold based on an R0 of 2.5 can be reduced from 60% of a population getting infected right down to 10%, depending on the quantity and distribution of pre-existing immunity among people, Gupta’s group calculated.