This means the mortality rate is dramatically less than previously stated.
What are you saying it was 'previously stated'? I haven't seen any clear statement of the case fatality rate. I've seen multiple, conflicting guesses. To say it was 'dramatically less than previously stated' doesn't mean much, when it's never been clearly or definitively stated to be any particular figure in the first place!
Copying my own post from a p&n thread, it appears from these figures that the case fatality rate is maybe 1.1%, which is what I vaguely thought it might be from the start...but it's still a long way from definitively established (and 1.1% is still ten times higher than seasonal flu):
So the worldometer site has a note saying
New York State Governor Cuomo said that preliminary findings from an antibody study conducted on 3,000 people at grocery stores across New York State found a 13.9% had coronavirus antibodies, suggesting a 13.9% actual infection rate statewide (21.2% in New York City), which translates to an estimate of about 2,700,000 actual cases in New York State (10 times more than the about 270,000 cases that have been detected and reported officially). Governor Cuomo acknowledged that the official count reported by New York State (which still is not including probable deaths as recommended by the new CDC guidelines) of about 15,500 deaths is "not accurate" as it doesn't account for stay at home deaths. Based on Worldometer's count (which includes probable deaths reported by New York City) of about 21,000 deaths and the 2,700,000 case estimate from the new antibody study, the actual case fatality rate in New York State could be at around 0.78%
It seems to me that the calculation of cfr there is not right. Because that's taking total deaths to-date and the estimate of cases right now at this moment. But there's going to be a lag between infections and outcome (death or recovery), which seems to be somewhere between 2 and 4 weeks. i.e. even if the virus magically stopped spreading today, some of those currently infected would still die over the coming month.
So, surely, what you should do is divide the total deaths so far by the full infection case count of 2 weeks ago, not the number now. Or, alternatively, divide the total death figure we have 2 weeks from now by the current infection count. As the NY deaths are running at 600 a day, then the more accurate calculation of cfr would be more like 29,000/2,700,000, i.e. 1.1%
Another point is, as the mayor himself pointed out, the prevalence calculation is based on testing people who are out-and-about, who would probably be more likely to be infected than those staying indoors, thus the prevalence figure could be an over-estimate. And finally New York has a younger population than many parts of the US, so would expect to have fewer deaths. So it's possible the death rate elsewhere will be higher than 1.1%. But it's all guesswork at this point, I think.