I also wanted to share an anecdote from NYC that I think drives home the “lack of data” and “denominator” issues.
An elderly person I know in NYC has a clinical diagnosis of COVID but is not sick enough to need to go to the hospital. Their general practitioner has told them to self-quarantine for 14 days and only go to the hospital if they get sicker. I confirmed with the doctor - and he does not report this case to the Department of Health – it is a conversation between doctor and the patient.
This means that there is no case reporting system currently in place in NYC. If they weather this at home and survive, they are not reported. If they go to the hospital they become a “case” whether or not they survive (denominator) and if they die they become a “death” (numerator).
If we don’t have widespread testing or reporting and surviving non-hospitalized cases (e.g. anyone who has COVID but is not going to the hospital) are not being reported as part of the denominator, the case fatality rate in NYC appears higher than it is including among the elderly.
For more on this read to the bottom of Helen’s article in STAT. which also describes some of the required conditions and pitfalls of how this is playing out in some of the countries that were “ahead of us” on the epidemic.