Here is the Bendavid and Bhattacharya WSJ piece that perhaps has had the biggest popular influence. They argue that many more people have had Covid-19 than we think, the number of asymptotic cases is very large, and the fatality of the virus is much lower than we think, perhaps not much worse than the flu. But their required rate of asymptomatic cases is implausibly high.
The best evidence (FT) for asymptomatic cases ranges from 8 to 59 percent, and that is based on a number of samples from China and Italy, albeit imperfect ones. Icelandic data — they are trying to sample a significant percentage of their population — suggest an asymptomatic rate of about 50 percent. To be clear, none of those results are conclusive and they all might be wrong. (And we should work much harder on producing better data.) But so far there is no particular reason to think those estimates are wrong, other than general uncertainty. You would have to argue that the asymptomatic cases usually test as negative, and while that is possible again there is no particular reason to expect that. It should not be your default view.
Marc Lipitsch put it bluntly:
The idea that covid is less severe than seasonal flu is inconsistent with data and with the fact that an epidemic just gathering steam can overwhelm ICU capacity in a rich country like Italy or China.
Furthermore, the “optimistic” view implies a much faster spread for Covid-19 than would fit our data from previous viral episodes, which tend to come in waves and do not usually infect so many people so quickly.
So I give this scenario of a very low fatality rate some chance of being true, but again you ought not to believe it. The positive evidence for it isn’t that strong, and you have to believe a very specific and indeed unverified claim about the asymptomatic cases testing negative, and also about current spread being unprecedentedly rapid.
Here is Tim Harford’s take (FT) on all this, he and I more or less agree.
By the way, Neil Ferguson didn’t walk back his predictions. That was fake news.
So we still need to be acting with the presumption that the relatively pessimistic account of the risks is indeed true. Subject to revision, as always.