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Abstract

As severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spreads, the susceptible subpopulation declines causing the rate at which new infections occur to slow down. Variation in individual susceptibility or exposure to infection exacerbates this effect. Individuals that are more susceptible or more exposed tend to be infected and removed from the susceptible subpopulation earlier. This selective depletion of susceptibles intensifies the deceleration in incidence. Eventually, susceptible numbers become low enough to prevent epidemic growth or, in other words, the herd immunity threshold is reached. Here we fit epidemiological models with inbuilt distributions of susceptibility or exposure to SARS-CoV-2 outbreaks to estimate basic reproduction numbers (R_0) alongside coefficients of individual variation (CV) and the effects of containment strategies. Herd immunity thresholds are then calculated as 1-(1R_0 )^(1((1+CV^2 ) )) or 1-(1R_0 )^(1((1+2CV^2 ) )), depending on whether variation is on susceptibility or exposure. Our inferences result in herd immunity thresholds around 10-20%, considerably lower than the minimum coverage needed to interrupt transmission by random vaccination, which for R_0 higher than 2.5 is estimated above 60%. We emphasize that the classical formula, 1-1R_0 , remains applicable to describe herd immunity thresholds for random vaccination, but not for immunity induced by infection which is naturally selective. These findings have profound consequences for the governance of the current pandemic given that some populations may be close to achieving herd immunity despite being under more or less strict social distancing measures.

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https://www.medrxiv.org/content/10.1101/2020.07.23.20160762v1

Appreciate the bold section but not sure my dumb arse still understands....

Are they saying herd immunity can start with as little as 20% of population infected?

Correct. Or less.

It's a consequence of heterogeneity.

As little as 10-20%...explained in the first few sentences.

Keegan has been providing many of these studies, discussions and models recently. All very interesting and compelling. I hope, hope, hope his finds turn out to be correct.

Keegan99 said:

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Quote:

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Abstract

... Individuals that are more susceptible or more exposed tend to be infected and removed from the susceptible subpopulation earlier. This selective depletion of susceptibles intensifies the deceleration in incidence. Eventually, susceptible numbers become low enough to prevent epidemic growth or, in other words, the herd immunity threshold is reached.

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Do they have any idea what makes people more susceptible or more exposed to be infected?

If people avoid large indoor gatherings where distancing can't be maintained and air circulation is poor does that help make people less susceptible to being infected?

If people wear masks when around others not in their own household to limit the radius of exhaled droplets/virus does that help keep those around them less exposed?

If somehow there was a way to help remove people from susceptible subpopulation by them being less susceptible and less exposed, then we might see these new daily case numbers come under control.

Fortunately there is a way and it is currently working. I call it adaptive behavioral herd immunity. It affects the curve the same as heard immunity but its done by adaptive measures that tries to deprive the virus of easy targets and subsequently lower the R-naught and therefore brings the contagion under control.

We are on the right track. Don't give this virus easy targets and it will be controlled well enough to avoid another shut down.

Joel Miller shared some thoughts on this preprint on twitter:

This is not what the paper is postulating at all.

It is not a claim about NPIs.

Keegan99 said:

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This is not what the paper is postulating at all.

It is not a claim about NPIs.

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Well, that is good then. I misunderstood what you were trying to say. I just get tired of people who think this is just going to disappear and use that hope as a reason not take reasonable steps to make people less exposed and susceptible. In my opinion hope isn't a good plan for keeping things open. People need to be taking reasonable active measures such as masking and distancing to keep this under control.

Quote:

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If people wear masks when around others not in their own household to limit the radius of exhaled droplets/virus does that help keep those around them less exposed?

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Re: Masks. Masks were never prevalent in Europe - Sweden in particular - and burnout still occurred in the ballpark of 20%. On the flip side, masks have also been mandated in LA since mid-May, and LA still saw a detected infection curve nearly synchronized to Texas. Maybe masks make a difference? Maybe they slow things a bit? But there is little empirical evidence on that front. Certainly nothing indicating that they fundamentally change the area under the curve.


The good news is Texas has about 385k confirmed infections. With a 6-9x multiplier for undetected infections, that means 2.5MM+ to 3.5MM+ have been infected, or in the neighborhood of 10% of the population.

Accordingly, detected infections have peaked and we're now on the downside of the Gompertz curve. Agreement is occurring among multiple indicators:

* Raw number of detected cases
* Decline in test positivity percentage
* Declines in new hospital admissions
* ER visits for COVID-like Illness

One way to look at this pre-print and its conclusions is as an individual. How can I make myself less susceptible to being infected by the virus when it is at its most virulent? If enough people decide to do this (and there are actually effective ways to do so), then society benefits. It starts with individual responsibility.

Thanks Keegan...

"Masks were never prevalent in Europe..."

This doesn't appear to be true as most hard hit countries required them coming out of their lockdown and countries who instituted them prior to a bigger initial wave avoided that tragedy...

https://www.aljazeera.com/news/2020/04/countries-wearing-face-masks-compulsory-200423094510867.html

Keegan99 said:

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Quote:

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If people wear masks when around others not in their own household to limit the radius of exhaled droplets/virus does that help keep those around them less exposed?

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Re: Masks. Masks were never prevalent in Europe - Sweden in particular - and burnout still occurred in the ballpark of 20%. On the flip side, masks have also been mandated in LA since mid-May, and LA still saw a detected infection curve nearly synchronized to Texas. Maybe masks make a difference? Maybe they slow things a bit? But there is little empirical evidence on that front. Certainly nothing indicating that they fundamentally change the area under the curve.


The good news is Texas has about 385k confirmed infections. With a 6-9x multiplier for undetected infections, that means 2.5MM+ to 3.5MM+ have been infected, or in the neighborhood of 10% of the population.

Accordingly, detected infections have peaked and we're now on the downside of the Gompertz curve. Agreement is occurring among multiple indicators:

* Raw number of detected cases
* Decline in test positivity percentage
* Declines in new hospital admissions
* ER visits for COVID-like Illness

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Really like informed discussions like this and I am in the group that believe that there is a 20% burnout rate due to infections, similar exposures to other corona viruses and innate T-cells with some people.

I like your current 10% number as well but suspect it may be higher due to the way it's burning through the population in the Valley.

Will be interesting in a year or two when the post analysis is done (in a less political climate) to really understand who was right and wrong.

Point being, burnout happened at ~20% without masks. The area under the curve wasn't changed.

Sweden focused on social distancing and reduced travel and Swedes largely complied.

https://www.government.se/articles/2020/06/social-distancing-and-markedly-reduced-travel-in-sweden/

Edit below from sidebar of above link:

Changes in Swedes' behaviour, 21 March7 June
88% Maintained greater distance to others

86% Washed hands more often and more carefully

82% Avoided shaking hands

66% Taken part in social activities outside the home to a lesser extent

64% Avoided public places where there are lots of people

Source: Swedish Civil Contingencies Agency

Yes, it has nothing to do with latitude and seasonality. It must be some vague compliance factor.

Humans are in control. It's not nature doing its thing.

Extremism regarding things I never mentioned is always a great fall back.

Let me speak more slowly, masks are working to a greater extent in Northern California where compliance is greater. You may not like that but the state numbers verify it. Masks worked when everybody wears them. The more that don't wear them, the less they work. It's pretty simple.

Wearing a mask in public is a minor inconvenience and one small thing we can do to chip in. Avoiding large gatherings also works as illustrated, well, everywhere. The resistance is mind boggling.

You have no idea why it's spreading more in L.A. than NorCal. You just made that up on the spot with zero evidence to support it.

AgE Doc said:

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"Masks were never prevalent in Europe..."

This doesn't appear to be true as most hard hit countries required them coming out of their lockdown and countries who instituted them prior to a bigger initial wave avoided that tragedy...

https://www.aljazeera.com/news/2020/04/countries-wearing-face-masks-compulsory-200423094510867.html

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some of those "green" areas, with more stringent mask rules have outbreaks worse than the US right now, specifically Mexico.

Firstly, just want to commend this thread for this not turning political! In.

Anyone have thoughts/opinions on infections rising in Spain? Not sure how many total infected they hit during their first wave (maybe failed to reach 10-20%?) but to me it just adds to the idea that there is nothing that can truly be done to substantially stop or prevent this thing. They've been locked down for months after a terrible infection, and even as they slowly let up, infections come back. I just don't see a way around it.

I do believe in the burnout of 10-20%. Maybe a little higher. What do Sweden's numbers look like? From my understanding they may have hit the burnout of this thing.

I've seen people comment that the current breakout in Spain is in an area not hit hard in initial outbreak...similar to here (NY vs TX)

Aggie95 said:

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AgE Doc said:

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"Masks were never prevalent in Europe..."

This doesn't appear to be true as most hard hit countries required them coming out of their lockdown and countries who instituted them prior to a bigger initial wave avoided that tragedy...

https://www.aljazeera.com/news/2020/04/countries-wearing-face-masks-compulsory-200423094510867.html

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some of those "green" areas, with more stringent mask rules have outbreaks worse than the US right now, specifically Mexico.

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Very true about Mexico. Mandates and compliance with mandates can be two different things. Similar to what Iran is dealing with regard to mitigation apathy.

Another things also to consider when looking at a countries risk for a really bad outbreak is to what degree do they have many people and multiple generations living in one home. Mexico has many multigenerational homes and to my knowledge no County is mandating masking within a household. The more the average number of people in a home the higher the R-naught would be expected to be.

bay fan said:

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GAC06 said:

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You have no idea why it's spreading more in L.A. than NorCal. You just made that up on the spot with zero evidence to support it.

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Actually I know many young adults in SoCal and It's pretty clear based upon social media what their habits are. I also know many of their contemporaries here in Northern California and I know their habits as well while you have just your opinions without any direct knowledge. Ignore the facts, honestly I could care less about people like you who think they know far more then they do and are simply disinterested in facts others can provide.

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Nobody cares about your theories based entirely on anecdotal evidence. Just stop.

And in case you haven't noticed, you haven't provided any facts.

Except Sweden (and perhaps more strikingly, the Diamond Princess cruise ship) stand to refute the theory that it's just some consequence of behavioral happenstance.

Your theorized chronology also seems dubious. You're suggesting that lockdowns occur, thus immediately and drastically altering behaviors, but infections still continue to rise until ~20% is hit. Yet only then there is some mysterious "behavioral change" and, despite subsequent relaxing of restrictions, no substantial reignition occurs?


You also completely ignore any existing immunity, such as from T cells.

This article by an epidemiologist is pertinent to the discussion in this thread...

https://www.google.com/amp/s/www.thejakartapost.com/amp/life/2020/07/13/covid-19-could-it-be-burning-out-after-20-of-a-population-is-infected.html

Just finished reading the pre-print. I'd probably characterize it a study in model fitting to real world data.

Overall it's an interesting study that tries to take into account the variability in the epidemic SEIR curves created by different levels of effectiveness of reducing contact (what they refer to as exposure or connectivity) and heterogeneous "susceptibility".

Reading critically, the modelers' forecasts rely on a reducing force they term Coefficient of Variation or CV. In effect the CV reduces the herd immunity threshold by reducing the efficacy of the basic reproduction rate of the virus (R0 or R-naught). The below chart shows herd immunity thresholds for different levels of R0, assuming the classical formula for herd immunity (CV=0), or external CV forces (CV=1 - 5). The report's conclusion of 10-20% herd immunity relies on one of the CV's to be between 2-5.



Basically the susceptibility CV looks at different variables' effects on the infection rate force multiplier. It's not super well defined how susceptibility is variable given this is a novel virus and theoretically 100% of people are susceptible, but the report alludes to age-related effects, immunity following asymptomatic transmission and maybe in a roundabout way mask-wearing(?). Either way, it assumes people who are "more susceptible...have a higher propensity to be infected [earlier] and thus are likely to become immune earlier," which felt like a little bit of a leap. Frankly this part had me thinking in circles because it seemed not well defined, so maybe someone here can follow up with what exactly they're trying to say.

Exposure was better described, basically the effect of non-pharmaceutical interventions (i.e. lockdowns, social distancing, etc) to reduce the overshoot of infection until a sufficient amount of the population has immunity to undermine the continued viral transmission.

In the main text the assumption was made these interventions would be ratcheted up over 3 weeks, sustained at maximum lockdown for one month and then progressively (linearly) relax over 4-6 months. The below charts on the left additionally include model runs for the scenario no interventions were implemented (black curves). The assumption is also made these interventions would be applied evenly across the total population. The report included a follow up look at what may happen if social distancing were imperfectly followed, but held over the same time period showing moderate second peaks in some cases (right chart):



The report further looked at sub-regions within the countries and how their individual dynamics, timing and herd immunity thresholds were different than the national total. One takeaway was higher density regions (cities) had higher thresholds than the countries at large, Spain for instance is below:



Interestingly, they concluded:

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Looking back, we conclude that NPIs [non-pharmaceutical interventions] had a crucial role in halting the growth of the initial wave between February and April. Although the most extreme lockdown strategies may not be sustainable for longer than a month or two, they proved effective at preventing overshoot, keeping cases within health system capacities, and may have done so without impairing the development of herd immunity.

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please do not quote bay fan in your replies...