Its fatality and infection rate - published that day in the Lancet - meant >500K deaths if left unchecked. And it could have been stopped in its tracks with far less economic damage than we face now.
I believe he realised he could not declare emergency measures without being asked to pause Brexit. I believe he chose to risk 0.5M British lives, to get his Brexit over the line. And everything since has been a post-rationalisation...