They're all wildly different populations within different environments and infection rates. COVID-19 infects with very stochastic clustering, making controlled

They're all wildly different populations within different environments and infection rates. COVID-19 infects with very stochastic clustering, making controlled experiments difficult. So one may even need global meta-studies with local control groups, for meaningful results. During an uncontrolled pandemic, you just don't have that luxury and enough degree of control.