Here’s the explanation at Covid.us.org: The Covid-19 Death Rate Has Dropped by 2/3rds. The comparison is weekly total of new cases to weekly total of new deaths. The new deaths is from a week 14 days later than the new cases week. That’s because it takes about 14 days or so from diagnosis for someone to die of Covid, if they are going to die. And that timing is of course an average.
If you were diagnosed with Covid in the week of April 5 to 11th (Sun. to Sat.), your chances of dying were about 6.88%. This uses the week beginning April 19th (14 days later) for the weekly new deaths data.
But if you were diagnosed in the week beginning June 14th, your risk of death was 2.21%. This uses the most recent full week, beginning June 28th, for the weekly new deaths data.
That is a difference of 3.11 times. So the risk in late June/early July was less than a third of what it was in April. And that is just over 2/3rds less risk of death.
The article here discuss why that is. And it is not all good news.
RLCJ
the reproach of Christ 
Couldn’t the seasonal nature of the virus and how it relates to vitamin D levels be either confirmed or ruled out by reviewing death rates by region? For example, Florida gets sunny weather much more consistently throughout the year than say, Minnesota. So if the death rate has declined in Florida, along with other states, then perhaps it’s reasonable to assume that treatment options are more effective or that even the virus itself is weakening which is the main cause of the decline of deaths rather than simply the summer season.
I’ve not seen a study yet of Covid deaths by State, taking into account sunny weather. That might be a useful study to do.