What is the logic of bringing together untested groups of people?

Source::
The National
What is the logic of bringing together untested groups of people?

I WAS evidently mistaken to think that Johnson’s urging of the public to go forth to the parks and spread Covid-19 was his last throw of the dice to generate “herd immunity” for the UK.

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The National
What is the logic of bringing together untested groups of people?