Yes. But according to my knowledgable source (daughter with a masters in Biology) the actual proportion who only get mild symptoms during the course of the infection is unclear.
But there do seem to be some, yes.
The data she says are at least fairly reliable are the percentage of closed cases (cases that have had an outcome) that result in recovery/discharge (79% worldwide) or death (21%). But that says nothing about the severity of the symptoms.
If 1 in 5 of those (on average) who get the virus, die from it, that's not reassuring.
It will vary of course. Older people may have a higher fatality rate than younger people. Those in so-called 1st world countries may have a lower one than those not. In my case, those two things might balance out.
In my head I am trying to reconcile that (high) 21% fatality rate with the (much lower) estimate of 2.5-4% that I read of elsewhere in different statistics.
I'm hoping there is an explanation that favours the latter. Something I'm hopefully missing.
You are missing
a) the fact that death closes a case quicker than a verified recovery. Say it takes two weeks on average for a person to die, but 3 weeks before another can be safely declared recovered after two subsequent negative tests and a few days without symptoms, and be released back into the general population without risking they infect anyone. As long as infections are rising exponentially, the recovered and dead population have different bases. Say there were 500 infections in week 1, 1000 in week 2, 2000 in week 3, and the actual mortality is 5%. By week 5, 25 of the cases from week one, 50 from week 2, and 100 from week three will have died (175 total) but only the ones from weeks 1 and 2 will have recoverd (475+950) - in that scenario, the
apparent mortality from dividing current deaths by resolved cases will be almost 11%. The actual ratio between the two values will depend on 1) how fast cases grow and b) how much longer it takes for the average recovery than for the average death, but you get the picture.
b) Another factor is that many cases go undiagnosed, and most of those that go indiagnosed are probably mild or even asymptomatic cases. Say only every other case but almost every severe case is caught, and every case that eventually results in death. With those assumptions, the apparent death rate would double yet again - because a recovered patient who was never diagnosed doesn't show up in the "recovered" figure.
So with these two factors, you already get from a 5% real case fatality rate to a 22% apparent rate as derived from (deahts/resolved cases). Add to that that due to limited testing capacities many countries preferentially test people known to be at risk, with young and healthy people often being bypassed for testing even when they show symptoms - they may be told to self- isolate just in case but won't be added to the confirmed cases tally, or to the confirmed recoveries later unless their condition deteriorates enough to require hospitalization at a later point. This means that the skew factor for the (b) part of the explanation might actually be much higher than 2.
The most reliable estimates I've seen that take this into account come up with case fatality rates between 0.5 and 1.5%, and probably closer to the lower end of that range. That's still a *lot* - influenza has about 0.1% and rarely infects more than 10-15% of the population in any one season (because there are vaccines, and because people's immune system has experience with similar strains from past years that make them partially, though not reliably, immune), while this thing, left to run its course, would probably infect 60-75% of the population before it retreats. So even under the most optimistic scenario, the final death toll would be around 20x that of a bad flu season.
All of that without a collapse of the health care system which inevitably pushes up the case fatality rate for this disease as well as producing collateral damage in the form of people dying from other preventable causes because we lack the resources to treat them during an ongoing pandemic. These two factors combine to make a 5-10-fold increase for an unchecked scenario plausible.