It is non-linear and non-robust system. The main problem is that mortality rate even among young people would become much higher as soon as hospitals become overwhelmed.

So you can not make any predictive conclusions based on current stats. Please look at any of the numerous models available online which account for all factors, and you will see how badly herd immunity idea would end for all age groups.

I don't see how this follows. The point the parent was making is that young people don't seem to need hospital care. Why would they suddenly start dying just because 'hospitals are overwhelmed'? They still won't need them...

Also, when hospitals are overwhelmed they stop all treatment. That young person who gets shot; has a road traffic accident; has a drug or alcohol overdose; has a problematic pregnancy and birth; has cancer -- all of these treatments become harder to provide.

Low risk doesn't mean no risk, and we see plenty of younger people in hospital, and in intensive care, because of covid-19.

ICNARC has a report that they update over time using data from UK intensive care units. https://www.icnarc.org/Our-Audit/Audits/Cmp/Reports

https://www.icnarc.org/DataServices/Attachments/Download/c5a...

Age at admission (years), number of people discharged alive (%), number of people died (%)

  16-39     240 (78.4)     66 (21.6)
  40-49     350 (73.2)    128 (26.8)
  50-59     568 (57.0)    429 (43.0)
  60-69     503 (41.8)    701 (58.2)
  70-79     293 (32.2)    617 (67.8)
     80+     57 (31.3)    125 (68.7)

Not taking any position nor bringing data in this debate, just pointing out the logic

But some jurisdictions are so locked down that there is virtually no spread at all, leaving the population highly vulnerable to a second wave.

Hospitals would only become overwhelmed if the virus were to spread rapidly among high risk groups. Not many low risk individuals will wind up needing hospitalization.

Lastly, it would be more constructive to the debate to link to data instead of asserting that data exist somewhere out there which support your point.

It's exactly what the model giving us 2.2M deaths in the US with strict quarantine was doing. Extrapolating the cruise ship case of 1% death to 80% of the US. Parent math is actually more reasonable.

When it returned the following year. It mutated to become much more dangerous to younger people.

[0]https://www.history.com/news/spanish-flu-second-wave-resurge...

Careful with that. There have been a number of deaths in younger age groups, statistically not as significant as those in older and at risk groups, but the risk is there.