hoakley November 2, 2020 General, Life, Technology

What’s the cost of acquiring herd immunity to Covid-19?

Many are proposing a way ahead with Covid-19 based on letting the infection spread naturally through populations to allow them to develop herd immunity. How feasible might that approach be in the population of England?

There’s a well-known target required for this to work in any population: the Herd Immunity Threshold (HIT), the proportion of the population who must have immunity before spread slows and stops. This depends on the R0 for the infection: HIT = 1 – (1/R0).

For flu, R0 is typically around 2, making HIT 50%. Half the population must have immunity to that type of flu before herd immunity cuts in and that epidemic stops as a result. Note this R0 isn’t the same as the effective R which varies in time and place.

Estimates for R0 for SARS-CoV-2 vary widely, up to 4, but it’s unlikely to be much below 2. So for natural herd immunity to take effect, at least half the population need to have had Covid-19 and currently be immune to reinfection.

Next we need to know what proportion of the population are currently immune to Covid-19. There are several studies covering England, including REACT which gives 6% in early July, and latest figures from blood donors give 5.7% to mid September.

So to reach HIT, at least 44% more of the population need to have Covid-19 and develop immunity to it. For England, that’s just under 25 million from a total of 56 million. We’ll give them 3 months to get infected and immune, generously 100 days.

That works out at a quarter of a million (246,400) fresh cases of Covid-19 per day on average, for a period of 100 days in England alone, to reach the HIT of 50% to provide herd immunity by natural infection.

We now need to work out the consequences of that sustained high rate of fresh cases. The percentage who require care in hospital varies widely with population, anything from 1-5% or more. That’s anything from 2,460 to over 12,000 cases each day.

Overall death rate (case fatality ratio) also varies. Worldwide it has been over 3%, and 6% in the UK first wave, but could be as low as 1% now. That means there’d be an average of 2,460 deaths per day for 100 days in England alone. Total dead 246,000.

Even halving these figures for the cost of attaining natural herd immunity in England to bank on a miracle, it would require daily hospital admissions of well over 1,000 for 100 days, and the deaths of 123,000 people on top of the 46,717 already dead.

More likely those figures would be worse still, with daily averages of 6,000 hospitalisations and 3,000 deaths, for a total of 350,000 dead. That’s in England alone, which doesn’t have that hospital capacity.

That’s why society has invested so heavily in immunisation, which can produce herd immunity with almost no need for hospital care or death. But some seem hell-bent on putting the clock back to before 1770 and killing people when there’s a better way.

Next time someone suggests just letting Covid-19 spread so we acquire natural herd immunity, get them to show you figures for its impact. If they don’t, then they’re just selling you worse suffering and death. There’s a special place in hell for them.

I leave it as an exercise for those in other countries to step through the same calculations to apply to their own situation. Looking from here it’d be no better.

A final note: models also show that vaccinating when the effective R is below 1 makes it quicker and easier to confer artificial herd immunity. So out of control infections will make it harder to vaccinate effectively too.

References:

https://www.the-scientist.com/features/why-r0-is-problematic-for-predicting-covid-19-spread-67690 on R0; REACT study https://www.medrxiv.org/content/10.1101/2020.08.12.20173690v2; Halloran ME et al 2009 https://www.springer.com/gp/book/9780387403137

This is a compilation of my Twitter thread.

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1

Michele Galvagno on November 2, 2020 at 10:04 am

Thanks for this very accurate reading.

I bring here the experience we’re getting from our Serbian relatives because—who knows why?—they are not appearing on any tabloid or newsagent. As you said, no country has a hospitalisation capacity able to stand this herd-immunity process, which means people in need would just die in their home or in front of First Aid in the cold, a scenario we possibly want to avoid. In Serbia people died to the thousands because they were lied upon the availability of beds in hospitals and they brought their dear ones there, just to know, a few days after, that they had died alone in a common room. This was the trigger that made the June 2020 protests erupt and which was, sadly, repressed in blood by the police and state army. We in Italy do not want a second Bergamo, where the only noise you heard was that of the armoured vehicles of the army bringing away thousands of coffins to be burned … in Germany, because we did not have place in our graveyards. Those who lost their dear ones never knew what really happened to them and probably never will… this is the greatest crime.

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- 2
  
  hoakley on November 2, 2020 at 12:51 pm
  
  Thank you.
  I think those responsible for this sort of slaughter should find themselves before an international court. It is a crime against humanity, as much as any war crime might be.
  Howard.
  
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  - 3
    
    Michele Galvagno on November 2, 2020 at 12:54 pm
    
    That would certainly be the case, but Serbia is not yet EU, USA want them to sign for Kosovo (drugs, bases, etc…), Russia has its interests there as well…
    Also, they justified it saying that the protesters started to burn the parliament, when truth is that insiders sent from the president started fires which then of course made the police cavalry come out…
    
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4

lestertwice on November 2, 2020 at 3:57 pm

I don’t like commenting about Covid-19, because I’m really sickened by the unspeakable wretchedness of most ruling classes in UE, to say nothing of USA, but I feel uncomfortable when reading about cost of acquiring herd immunity. I mean we should not even accept to discuss what is most likely a fraud. Maybe I’m linking to sources you already mentioned, I confess I didn’t read most of your articles about Covid-19 in UK. I’d like to recommend though a freely readable paper on Nature eloquently titled “The false promise of herd immunity”:

https://www.nature.com/articles/d41586-020-02948-4

To quote the gist of it: “We have never successfully been able to do it before…”

I’ve found relevant also this other article:

https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)31924-3/fulltext

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- 5
  
  hoakley on November 2, 2020 at 5:51 pm
  
  Thank you.
  Those are two good articles, although The Lancet looks a little like post hoc political correctness. I trained in medicine in the late 1970s, and the term herd immunity was in widespread use then. It had no sinister connotations until 2020, when it appears to have been hijacked and used in a different sense, to apply only to the resistance of a population which had been acquired by infection. Until then, it had been entirely neutral, and was often used in the context of the resistance resulting from immunisation. Now many are trying to distance themselves from the term, which is also defined in most books on epidemiology and infectious disease, again in purely neutral terms.
  I felt that the excellent Nature article merits the impact of real numbers and a real population. Unfortunately those peddling their poisonous ideas are often orators who know how to prey on the less well-informed. They’re always vague, and hate having to back up their claims with hard reality like numbers. They are also adept at selecting (opposing) worst-case predictions and using them out of context, which is why I wanted to give numbers for their best case, which their proposals could never achieve.
  Personally, I think that advocating policies which are universally agreed would lead to a great many more deaths should be a crime, just as incitement to violence or hatred. It is after all the ultimate violence towards your fellow humans, knowing how many you’d kill.
  Stay safe,
  Howard.
  
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  - 6
    
    lestertwice on November 2, 2020 at 6:26 pm
    
    I totally agree, you couldn’t have made it clearer. As for The Lancet’s article, I didn’t like too the unnecessary quibbling about political correctness, but I found interesting the historical perspective. I think that it’s systematically neglected nowadays even though it may often help as much as numbers to understand things. Or maybe it’s simply that my mother had diphtheria when she was a child and almost died.
    
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7

Reading: What’s the cost of acquiring herd immunity to Covid-19? – Morgan's Log on November 2, 2020 at 4:17 pm

[…] From What’s the cost of acquiring herd immunity to Covid-19? […]

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8

Carol on November 2, 2020 at 7:45 pm

Nice analysis. Unfortunately, I think you’ll lose most anti-maskers and anti-vaxxers before they finish the second paragraph, which is part of our problem here in the U.S.

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- 9
  
  hoakley on November 2, 2020 at 8:20 pm
  
  Thanks.
  Believe it or not, in the UK we actually have a ‘political party’ being formed to oppose measures such as lockdowns being used to control infections. That was the spur to my writing this.
  Howard.
  
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10

Jeremy on November 3, 2020 at 7:18 am

Thank you, Howard, for your lucid and helpful explanation.

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11

Alex on November 3, 2020 at 10:54 pm

I think your numbers are much too low. You’re looking at fatality rate as a percentage, but it’s not anywhere close to a linear function. We’re severely constrained by many factors, and at each threshold we hit, the fatality rate will go up dramatically.

You mention “hospital capacity” briefly in passing. Managing a pandemic is *all about* capacity.

Take ventilators, for example. The most recent numbers I can find suggest that around 5% of COVID-19 patients require mechanical ventilation. If you’re getting 250,000 new cases per day, then perhaps 12,000 will require ventilators. NHS has 30,000 ventilators in total, so we’ll run out before lunch on day 3. The other 97.5 days of this experiment, those 12,000 will go straight to the morgue. (Assuming you have enough morgues to handle 12,000 extra bodies per day.) And 30-50% of those put on ventilators end up dying, anyway.

Or look at beds. There’s only 150,000 hospital beds in the country. With 10% of patients requiring hospitalization, you’re out of beds by the first weekend. Surely the fatality rate won’t remain constant if we’re sitting people in a chair in the corridor, or sending them home?

Or look at the number of healthcare professionals available. There’s only about 100,000 doctors and 300,000 nurses in the NHS. Is that enough to care for all the new cases, even if 90% are asymptomatic?

Based on some back-of-the-envelope calculations, I don’t see any way to achieve “natural herd immunity” in the UK without costing at least a couple million lives.

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- 12
  
  hoakley on November 4, 2020 at 9:38 am
  
  Thank you.
  I am deliberately using ‘best case’ estimates, as I have noted above. The problem with countering these arguments is that those who put them always use worst case estimates out of context for alternatives. If you provide them with likely or worst case estimates, they turn them to ridicule. It’s better to use low estimates to demonstrate that even those wouldn’t be feasible as being claimed.
  I’m also making the assumption that, under a plan to allow Covid-19 to spread and so bring herd immunity, it remains under control so the numbers don’t grow exponentially, which they would if it were given free rein. That would still require some lockdowns, and impose considerable economic and social hardship.
  One common claim being made is that the proportions requiring hospitalisation and ventilation are based on the first wave, and that the second wave is resulting in much lower demand. I think that’s false, and the coming weeks will demonstrate that these have improved, but not by much. Hence I am deliberately using low end and optimistic figures throughout to show how those claims aren’t even consistent with more radically low views on hospitalisation, ventilation, and fatality ratio.
  All of this also hinges on how effective and lasting naturally-acquired herd immunity might prove. The evidence so far isn’t bad, with the numbers who suffer second infections remaining low, but most who had Covid-19 in the first wave are still within six months of their first infection, and this is also still early into the second wave, in all probability.
  Howard.
  
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13

G.J. Parker on November 3, 2020 at 11:56 pm

Thanks for pointing out the absurd notion of trying to get to natural herd immunity.

HIT = 1 – 1/R0 is when the rate of new infections hits zero- that is, when rate of new infections stop increasing. So even if we hit that, the number that will be infected will continue to go up, just at an ever decreasing rate. So even if you’re prepared to have 50% of the population infected, you’ll end up w/ something higher (exactly how much higher depends on other parameters, such as how long someone is infectious).

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- 14
  
  hoakley on November 4, 2020 at 9:42 am
  
  Thank you.
  Actually, what happens when a population reaches HIT is most dependent on the instantaneous R value at that time. If the R value at that time is still over 1.5, then the number of cases will overshoot and then suddenly fall back. If the R is much less than 1, then case numbers should fall rapidly.
  Howard.
  
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  - 15
    
    G.J. Parker on November 4, 2020 at 2:34 pm
    
    Thank you. Yes. I took the simple case that R=R0.
    
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