
The disease-induced herd immunity level for Covid-19 is lower than the classical - 2a0c40
https://arxiv.org/abs/2005.03085
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medymed
It seems like their simulated model makes certain members of a population less
‘active’ and so at less parametric risk of infection/transmission than in a
model where everyone is active. This is useful, and/but doesn’t appear to be
Covid-19 specific.

~~~
SpicyLemonZest
It isn't, and the same phenomenon has been observed in other cases. The 2009
swine flu for example had R0 estimated between 1.4-1.6, yielding a theoretical
herd immunity threshold of 29%-33%, but estimates of how many people were
actually infected range from 11%-21%.

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claudeganon
All this talk of herd immunity is not terribly helpful because we don’t know
what “immunity” means in practical terms.

The duration of immunity for other corinaviruses varies quite a bit, the
disease has the potential to cause permanent respiratory damage, with
uncertain consequences for the possibility of reinfection, and we’re still
seeing novel symptoms arise as in the case of the children with Kawasaki-like
inflammation in NYC.

I get the strong desire to return to a normal state of affairs, but paving
over reality with scientific notions, deployed pseudoscientifically, does very
little to get us there.

~~~
SpicyLemonZest
We know that the vast majority of people will recover fully, and after they
recover they'll be immune for a long time. Media reports have severely
overstated the uncertainty here; if coronavirus can indeed trigger Kawasaki
syndrome, it's only in the same sense that acetaminophen can cause liver
damage.

~~~
amiga_500
I've seen very different information on how long "for a long time" is. Even
just writing "for a long time" makes me wonder, if you have a range, why not
give it?

The figure for immunity duration at say 95th percentile seems to be the key
figure for herd immunity being viable or not, at least to my layman eyes.

~~~
SpicyLemonZest
It's hard to put a concrete number on how long immunity lasts, because
immunity isn't a binary process. All the experts I've seen discuss the issue
have said we can expect recovered people won't catch it again this calendar
year.

~~~
timmytokyo
That's just speculation because no one has had the virus for a year.

Please stop adding certainty where it is unwarranted. Either supply citations
to expert authorities, or be quiet. Otherwise you're just spreading
misinformation.

~~~
xamuel
Wouldn't this kind of reasoning apply to every new virus, including new flu
viruses that pop up routinely? A new virus pops up and appears to be just a
routine flu, but technically we can't know about its long term side effects
until many years later. So why don't we apply this same reasoning and lock
everything down routinely whenever any new flu virus arises?

It seems to me that you're right, we can't know for certain until many years
later, but in the meantime the world goes on, we can't all just cryogenically
freeze ourselves. The best we can do is make educated guesses based on
previously existing coronaviruses.

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mrfusion
My question is what R0 looks like in a city with mass transit vs without. The
subway in NYC is huge and puts so many people together in close contact every
day.

Is it outrageous to think R0 could be 3.5 in NYC but 1.5 somewhere else?

~~~
mannykannot
I think that, in the terminology of the paper, a higher proportion of New
Yorkers would be considered 'high-active' compared to, say, LA, and perhaps
there is a level that you don't even see in less crowded-mobile societies. I
would guess it is helpful to keep the biological and social factors separate
when modeling what is going on.

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lostlogin
> The classical herd immunity level hC is defined as hC=1−1/R0

How do you use this in places where transmission is less than 1? New Zealand
now has a transmission rate considerably under 1, but this seems to break the
calculation.

Does this mean that a reproduction number under 1 doesn’t need herd immunity?
We do, because we can’t stay locked down indefinitely, and presumably our rate
will go up when we relax restrictions.

~~~
jaynetics
R0 is the reproduction rate without any countermeasures.

~~~
lostlogin
Thanks, this is the key.

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octonion
This is also true under much weaker assumptions. Here's a simple example
calculation. Say 50% of a population has R0=1.5, 50% has R0=2.5; the average
R0 is 2. Then the required herd immunity is (2-1/1.5-1/2.5)/2=0.47, below the
classical 1-1/2=0.5. That this always true for any number of independent
subpopulations is a consequence of the harmonic-arithmetic mean inequality.

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code_duck
"lower than the classical level" would be a nice way to make the submission
title make sense and be a complete sentence.

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throwaway122378
Can anyone explain this in layman’s terms

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Ciantic
The paper says that herd immunity can be achived easier when the disease
itself does the spreading. Because in "classical" herd immunity the
vaccination does not target the active population so well. Instead when the
active population gets the virus naturally it will produce stronger herd
immunity.

~~~
e40
_Basically swedish model may be the right one._

What is that again?

~~~
Ciantic
Okay I removed that part, but Tegnell has now and then suggested the Swedish
model is "herd immunity" and then sometimes suggested it's not.

It does indicate the Swedish model could work, because the paper says it's
just 43% with "disease-induced" herd immunity. And according to Swedish
authorities 25% of Stockholm population already has antibodies.

~~~
Thlom
As I understand the Swedes, Herd immunity is not the goal they are working
towards, but a possible bi-product of their strategy. The strategy as such is
to keep the curve low enough, pretty much the same as everywhere else. They've
just used less extreme measures to achieve this because they don't think it
will matter in the long run.

~~~
stareatgoats
That is, if you want to give them credit for any strategy at all. And no, the
curve is not the same as everywhere [0].

Being a resident Swede, my lasting impression is that the so-called "strategy"
has two legs (can be backed by a long list of official statements) 1: hope
that the virus does not hit, and if it does, that it is not necessary to do
anything and/or it is already too late anyway and that hopefully it will
disappear through herd immunity. A hope that has been expressed up to twice
weekly for the last 2 months or so, while the number of deaths go up and up.
And 2: in a manner typical of governmental agencies everywhere: never ever
admit to any mistake or error in judgement. Like admitting that the
recommendation for the public not to use face masks was wrong, and was brought
on by secondary concerns (over availability in hospitals). Even now (as of
yesterday) the official stance is that using face masks might increase risks
and efficacy is not backed by "scientific proof".

The paper should probably be seen in this light. The main author of the paper
Tom Britton forms together with Giesecke and Tegnell the "herd immunity"
triumvirate that is responsible for the Swedish mess that currently renders
Sweden a place among the top 10 countries with most fatalities per million. At
the face of it this paper just seems to confirm that protective measures
actually work. But by shining the light the herd immunity aspect of the
equation he makes it sound like they were right all along.

[0]
[https://mackuba.eu/corona/#compare?val=d&align100=1&pop=1&c=...](https://mackuba.eu/corona/#compare?val=d&align100=1&pop=1&c=se,fi,no,tw,us)

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mrfusion
This is great news!

~~~
permalac
Without peer review this is weak.

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rossdavidh
Interesting that it seems to suggest there is an optimal amount of
distancing/lockdown/etc. to lower the peak, but avoid a big "rebound". I don't
see a lot of discussion about how to find that optimum amount.

It has occurred to me that the planet is essentially conducting a gigantic
experiment in virus evolution, by creating a novel viral selection environment
never seen before. For example, how does it impact a virus if the strains most
likely to send the host to the hospital, actually spread more than the strains
most likely to be asymptomatic, because even asymptomatic hosts are in
isolation?

Unless there is a vaccine or cure coming soon (there isn't), then the end
result must be herd immunity, no matter what the policy. It would seem that we
should be considering how to select for the healthiest segment of the
population to provide that herd immunity, rather than trying to isolate as
much as possible in the vain hope that it will die out prior to that point. If
that was every possible, it has long since spread way too far to expect that
to be possible.

There might be cases (e.g. MERS) where you can stomp out a virus before it
spreads enough to cause herd immunity, but we have long since passed that
point. I don't think most people (including policy makers) are thinking about
the endgame properly (as this paper does).

~~~
macintux
As far as I can tell, there is one big unknown that makes planning
particularly difficult: What are the long-term health effects from being
infected?

If it turns out that some significant percentage of the population is going to
have a negative impact over the course of their lives from this, should we
continue to hunker down until a vaccine or better treatments are found?

~~~
irq11
This is not an unknown. We have very good data on outcomes. The media has been
diving ever-further into niche stories of bad outcomes in order to drive
clicks, but the numbers of cases driving these narratives are vanishingly
small.

For the _vast majority_ (>98%) of those who are under 60 and otherwise
healthy, this disease is on par with the flu.

Even amongst those who are more vulnerable, the rate of long-term sequelae is
low. It’s the people who end up in the ICU who are experiencing the most long-
term effects, and those people are a single-digit percentage of a single-digit
percentage of those infected.

~~~
macintux
Quote from a virologist who caught it:

> Many people think COVID-19 kills 1% of patients, and the rest get away with
> some flulike symptoms. But the story gets more complicated. Many people will
> be left with chronic kidney and heart problems. Even their neural system is
> disrupted. There will be hundreds of thousands of people worldwide, possibly
> more, who will need treatments such as renal dialysis for the rest of their
> lives. The more we learn about the coronavirus, the more questions arise. We
> are learning while we are sailing. That’s why I get so annoyed by the many
> commentators on the sidelines who, without much insight, criticize the
> scientists and policymakers trying hard to get the epidemic under control.
> That’s very unfair.

[https://www.sciencemag.org/news/2020/05/finally-virus-got-
me...](https://www.sciencemag.org/news/2020/05/finally-virus-got-me-scientist-
who-fought-ebola-and-hiv-reflects-facing-death-covid-19)

~~~
irq11
Yeah, I read that anecdote, too. Saying “many people” is meaningless. A tenth
of a percent of a million people is a thousand people. That’s “many people”,
but the rate is quite low.

Facts: in the US, the hospitalization rate for covid is around 4.6 per
100,000. It’s half that for people under age 50. The rate of serious disease
is very, very low...and this includes high-risk groups:

[https://www.cdc.gov/mmwr/volumes/69/wr/mm6915e3.htm](https://www.cdc.gov/mmwr/volumes/69/wr/mm6915e3.htm)

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dr_dshiv
So, when active spreaders do most of the spreading, when they have herd
immunity, there is a disproportionate affect on the overall spread.

Meaning, let the young and stupid go out and play, it will allow for herd
immunity at a lower rate than the typical randomly distributed rate.

