John W wrote:
>I don't really get that. If everyone in the UK were infected on the same day, then, assuming a CFR of 2%, some 1.32 million (about double the 'normal' UK annual death rate) would die on/around the same >day
>(~20 days later, I think), with few/no Covid-19-related deaths thereafter, would they?
Indeed it would be something like that, assuming a CFR of 2%. However, I was only using this awful scenario to try to demonstrate what David S was trying to say (as I understand it). If the CFR is 2%, and everyone got the infection at the same time, then you're right, about 1.32 million would die. My 22,400 a day was an average over the period until everyone who was going to die, dies, and if that length of time is 20 days, then 1.32 million deaths is about 66,400 a day average over that period. That's still a lot more than 22,400. But the issue is the difference between the CFR you assumed (2%), what the CFR would be if this awful thing happened (a lot more than 2% for obvious reasons because of utter collapse of society), and what I was trying to demonstrate, the CFR implied by the figures David S based his calculations on, if everyone in the UK got the disease at once. This last number isn't realistic at all because it only applies in the awful impossible scenario. But you can work out what David's figures imply using the Infection Fatality Ratios in the last column of his Table 1. If you calculate a weighted average of those, using the actual proportions of people in the UK population in the age groups, that's what those figures imply the CFR would be if everyone in the UK was infected (not necessarily at once). That weighted average is 1.23%, a lot less than 2%, a possible reason for the difference being that the rate of being infected varies with age group and is lower in the younger age groups who have a lower fatality rate in David's table (which is based on what was used in the Imperial College model). So if everyone was infected, the number of deaths would be 1.23% of 66 million, or about 812,000. Divided by 20 days that's 40,600, still bigger than my 22,400, but not so much bigger.
What this implies, I think, is that using a ridiculously horrible scenario, to try to illustrate what David meant by a year's mortality risk into a few weeks, probably wasn't a good idea. (But the figures are roughly in the same ballpark, if you use the CFR for the entire UK population implied by the figures Imperial used for specific age groups.) Two points stemming from that. First, I think it was important that David stressed that he's only talking about the extra risk from SARS-CoV-2 infection during the time a person is infected, which will be a few weeks at most till recovery or death, so though the measure of risk comes out about the same as a year's worth of "normal" risk of death, the time that one is at risk is a lot different. Even if my attempt to get that across with some more numbers failed, I think it's an important concept. COVID-19 isn't a chronic disease (not as far as we know at least). Several people that have commented (including on the blog post) don't seem to have got this point. Second, the fact that the numbers imply that the CFR is a good bit less than 2%, if the risk of infection is the same for all age groups, means that either the 2% will prove to be too high, or more likely that it will become even clearer than it now is that the chance of getting the infection at all is not the same across all age groups.
Best wishes, Kevin
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