“We didn’t shut down the world then. Why now?”
There’s herd immunity with influenzas because some strain has gone around every year from time immemorial. And we have vaccines to help break the chain of transmission.
There was close to zero herd immunity when Covid-19 struck because the only other known SARS outbreak was the one in 2003. That SARS outbreak killed over 8% of confirmed cases. That’s all the data that CDC had to work with.
Where did you do your post-doc in immunology, perfesser?
Study: A majority of the population may have 'some degree' of preexisting immunity to COVID-19