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How we calculate the magic herd immunity number (it's possibly much, much lower than you might have heard). Why have some models failed us so spectacularly?
I FEEL HERD
Herd immunity is the magic percentage of the population which needs to be immune (brick wall, baby) before transmission entirely grinds to a halt. Very, very important: whatever the threshold number is, as you approach it, transmission is slower and slower and harder and harder and R0 drops and drops. It's more like molasses freezing, versus switch-flipping.
It's different for every pathogen, and can be calculated quick n'dirty style (the way modelers seldom like it) as follows:
Herd immunity = 1 - 1/R0
So, if your disease makes four new cases for each infection, the herd immunity threshold (HIT) is 75%. For measles, with an R0 of 16, it's 95%. For COVID-19, assuming R0 really is 2.2, it's about 55%.
But. BUT.
That's quick n'dirty. If that's all there was to it, modeling infectious diseases wouldn't allow us to buy our private jets and islands, etc.
What goes into R0, exactly? At the end of the day, it's whether or not an infectious person can sync up with susceptible, unwitting dupes into whom the virus can jump. And the problem with the quick n'dirty is that the model assumes ANYONE who hasn't been infected with SARS-CoV-2 is EQUALLY susceptible.
Newborn babies, chemotherapy patients- are they the same as the people whose pictures they put on vitamin bottles, or Chuck Norris?
Enter a multi-institutional modeling group from Oxford, the NIH, Portugal, Brazil, Edinburgh, etc (1). They make several very sound assumptions:
- the most susceptible people will be infected early on (i.e. nursing home residents, our canaries in the coal mines). Therefore the population will be somewhat depleted of them later in the epidemic.
- with COVID, you either die or you recover.
- individual variation in susceptibility is modeled continuously [for stats people suspicious about categorical variables]
- Social distancing is still the best way to reduce transmission and infectivity.
- After coming up with their clever equations, they fit the model on Austria and Italy's epidemics, and eureka! it predicted things perfectly!!
Conclusion: as the epidemic whirls on, the effective HIT drops dynamically, down-- at the end-- to only 10%.
And-- for everyone worried about whether or not effective first wave suppression has just left everyone vulnerable, and implies a nearly-as-big second wave once distancing is reduced: the model nicely predicts that removing the "highly susceptibles" from the population in the first wave makes the second wave likely rather muted.
THE MODEL SPECIFICALLY PREDICTS THAT AN IMMUNE FRACTION OF AS LOW AS 20% MIGHT SURPASS THE EFFECTIVE HIT.
This is why we can say, with some degree of hope, that NYC's 21% serosurvey antibody results might really, truly, be substantially close.
It's also why we can reasonably hope that places with smaller epidemics and lower prevalence might have muted second waves.
THE US IS NOT EXACTLY A MODEL NATION
Feeling frustrated with models? Yes, it's true, they set unrealistic expectations for appearance and hoard all the celery. Ah, the other kind? The one with numbers? Well, that kind is still very, very valuable.
Models are like judgy strangers on the internet: it's all in the assumptions. Their core parameters (called "assumptions") are as important as the way the variables are arranged in the equations.
And remember the more data which rolls in, the better the assumptions and the tighter the fit. Rather like quarantine jeans.
So why did some models overshoot, and some under-call? What do we know today that we didn't know 3 months ago?
- Asymptomatic transmission drives at least half of all transmission events [NOT assumed in the "cases are the cases" models]
- Some asymptomatic transmitters eventually become symptomatic [NOT assumed in the rosiest 'Big Denominator' models]
- The confirmed case count, especially in the first 2 months of the epidemic, was essentially a work of fiction and underestimated true cases by a factor of at least 10- potentially 50-80 [models like IHME took the US and international case counts as gospel]
- Deaths have been missed and under-counted [NOT assumed by basically everyone, since deaths are the most reliable number]
- The US is substantially sicker than the first nations on whose epidemic curves we modeled, so more people died than expected
- The upslope of the curve is much steeper than the downslope, where case counts linger like bad party guests (China's- our first example on which to model- was artificially ski-slopey).
And we know so, so much more about the basic pathology of the virus, and our immune response, and also about meth-addicted private zookeepers.
SOURCE
M. Gabriela M. Gomes, Rodrigo M. Corder, Ricardo Aguas et al. Individual variation in susceptibility or exposure to SARS-CoV-2 lowers the herd immunity threshold. https://doi.org/10.1101/2020.04.27.20081893
MEDRXIV.ORG
Individual variation in susceptibility or exposure to SARS-CoV-2 lowers the herd immunity threshold
https://www.foreignaffairs.com/articles/sweden/2020-05-12/swedens-coronavirus-strategy-will-soon-be-worlds?fbclid=IwAR3LU8dNdxPHPf6sCB9prHBwiheOT5lE0szDi2_CS24kQYYzT-hqp221YiA
FOREIGNAFFAIRS.COM
Sweden’s Coronavirus Strategy Will Soon Be the World’s
Thank you President Trump for the frontline worker fly by!
Great pandemic info for your education and questions
https://icimed.com/icim-expanded-position-statement-on-2020-coronavirus-covid-19-outbreak-with-references/
ICIMED.COM
ICIM April EXPANDED Position Statement on 2020 Coronavirus (COVID-19) Outbreak (WITH REFERENCES) - International College of Integrative Medicine
Here is a well done educational video to inform your self on the current Cov-2-SARS virus, your immune system, vaccines and government response.
https://fb.watch/28wAT6uHgU/
Unlocked posted a video to playlist Unlocked 1 on 1 Interviews.
November 19, 2020 at 4:33 PM ·
In this exclusive film, former Pfizer Vice President Dr Mike Yeadon discusses his thoughts as to why the lockdown was a mistake, and why the government strategies to manage the pandemic are only making things worse.
Why Exersice is important. It strengthens your immune system!
https://www.frontiersin.org/articles/10.3389/fimmu.2018.00648/full
FRONTIERSIN.ORG
Debunking the Myth of Exercise-Induced Immune Suppression: Redefining the Impact of Exercise on Immunological Health Across the Lifespan
What kind of public health system bemoans the fact that 40 percent of the population is so unhealthy that they are at higher risk of developing complications from the coronavirus and then shutters workout facilities (and even at points boardwalks, parks, etc.)? A coercive state that truly cared about its people would have forced them to exercise instead of shuttering gyms, walking paths, and bike trails!
