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Or better framed: Why are children not developing as severe reactions from COVID when they tend to fare much worse from seasonal flu?

Is the answer something along the lines of: Those that don't die from seasonal flu gradually develop immunity over the course of their lives thus protecting them through middle age. The effect of the protection gradually succumbing to the cumulative effects of aging.

But that doesn't explain the relative benign impact of COVID.

I'm not a health care worker, but am just curious about what is going on here. I'm half expecting the answer to be: It isn't known yet.

If it is the case though that the mortality risk from COVID-19 has an inverted L shape, with highest rates in older people. And if the flu is more like a U shape, with highest mortality seen in the youngest and eldest. Then what reason is there that children aren't hit harder by COVID-19?

Edit:
A quick look at the CDC website (1) suggests that the presumption may still hold, though I note the data table to be published November 22, 2021 so I'm not sure how much influence Omicron is having to those numbers. However at that time both the hospitalisation rates and Mortality figures display the heavy skewing towards higher numbers older aged patients. In contrast data from the 2017-2018 flu season (2) shows the distinctive U shape in hospitalisation rates. I think is a reasonable reference season because it's relativity recent and unlikely to be influenced by COVID effects. I do note that child mortality is not exceptionally high, relative to other cohorts, but the high hospitalisation rates at this time suggests to me that is as much to do with the intervention as anything else. Hence I think it is still valid to ask why the discrepancy with COVID? What is special about this virus and the mechanism of it's effect?

Additionally, however, it appears that in the most recent data available on the CDC's COVID NET reveals an uptick in hospitalisations in the under 4 yo cohort in line with a decline in most other age groups (as at Jan 1, 2022). A number of possible factors spring to mind, namely vaccine distributions in the older cohorts, and the relative impact of the omicron variant being different to previous strains.

The recent uptick in the group I had initially thought to be somewhat protected got me thinking about a possible explanation. My current working hypothesis has a couple of premises:

  1. The dominant spreaders relevant to SARS-COV2 during it's first waves of evolution where adult hosts (assuming adults represent the bulk of social mixing)
  2. Children have (perhaps) subtly different physiology from adults and the traits that made the original corona virus variants deadly were at first optimised for an adult focused transmission route.
  3. Despite not necessarily being tuned for transmission through the young, over time some virus does evolve to become better adapted to this route, in part because some virus is transmitted this way. There are no doubt plenty of ways this could happen (we are talking about at least two complex systems interacting), and no doubt a number of beneficial mutations result in poorer outcomes for host children.

Of course this is just a working hypothesis. But I thought I'd share it while I try to look for relevant data on what is actually happening here.

Edit 2:

So after a brief look over the wikipedia articles for influenza (3) and COVID-19 (4) it is not a whole lot clearer why there should be a difference in how they affect children. At least in the case of influenza it is well known that children who have not been exposed to multiple exposures have weaker immunity and consequently more severe symptoms. This simply ramifies the question in my view. Particularly in that COVID-19 appears in every relevant other dimension to produce more severe symptoms.

1: https://www.cdc.gov/coronavirus/2019-ncov/covid-data/investigations-discovery/hospitalization-death-by-age.html
2: https://www.cdc.gov/flu/about/burden/2017-2018.htm
3: https://en.m.wikipedia.org/wiki/Influenza
4: https://en.m.wikipedia.org/wiki/COVID-19

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    You write "...children not developing as severe reactions from COVID when they tend to fare much worse from seasonal flu?" Perhaps I missed it -- what is the evidence for this claim?
    – Armand
    Jan 10, 2022 at 9:20
  • Forgive my sloppy wording, I'm basing this on the hospitilisation rates per capita. I'm not presuming that any individual case is better or worse than any other, more wondering about why it is on a population level, on average kids get hit harder by flu (on hospitilisation rates)
    – Andrew Micallef
    Jan 10, 2022 at 11:03
  • Given the disruption to children's lives (e.g. remote learning), the stronger focus currently on preventing Covid, and the greatly differing Covid strains and exposure levels, at this point it's comparing apples to oranges -- we don't have data from comparable populations and exposure conditions.
    – Armand
    Jan 10, 2022 at 21:42
  • So I guess at present you might say it is just a quirk of the data, most likely due to the exposure conditions (amongst other factors). And then I guess looking forward, the question is unanswerable, and may still be based on an invalid premise.
    – Andrew Micallef
    Jan 11, 2022 at 3:40

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