The common cold versions are in the alphacoronavirus genus. COVID-19 and SARS-Cov are betacoronaviruses. Someone would have had to have SARS or had a vaccine against that to already be immune to this one.
Almost a third of the cases in Missouri are under age 40.
To my knowledge, there has been no sustained or urgent work on a SARS vaccine since 2004.
In any event, the resistance of young people in King County (Seattle) to COVID-19 is still remarkable.
The COVID-19 outbreak at the Kirkland, WA, nursing home was one of the most virulent and destructive medical events in recent American history. At least 35 residents died, and almost 100 staff were infected during a two week period.
But, less than three weeks later, high risk individuals in King County are testing positive at a 13% rate (compare almost 40% for peak seasonal influenza), and our Case Fatality Rate is 6.4%, almost one half of the Italian rate.
My theory - COVID-19 has already mutated into a much more benign disease in King County.