What if there already is a large latent-immune population?
Some scary numbers are out there, and some of them don’t make sense; one plausible explanation would be that milder forms of Coronavirus confers temporary immunity; there are policy and macro-economic ramifications.
The “known” numbers: mortality rate seems to be 1% amongst cases that test positive in a working healthcare system and ~5% amongst cases that test positive in a non-working/overwhelmed system.
Meanwhile, policy-makers and epidemiologists are saying: it is here; it is novel, so there’s no immunity; and it is sufficiently transmissible that one would expect that overall 40% — 70% of the adult population will get it over 12 to 18 months.
We also roughly know that of those that are positive for the Coronavirus 14% are hospitalized, 5% require ICU for about a month.
So it’s very easy to get very scary with all of this:
There are 100 thousand ICU beds in the US; even if all of those beds were unoccupied and we get all of the COVID-19 patients into the beds there’s only enough room for 2 million cases in a given month.
BUT the experts are telling us that having 50% of adults get it over 12 month is nigh inevitable.
That sounds a lot like 10+ million cases a month, which sounds a lot like the hospital system getting massively and completely overwhelmed.
Multiply number of adults presumed to get it by mortality rate in the event of an overwhelmed hospital system and it sure sounds like we need to start conducting a massive unprecedented hospital build-out.
The fear is that in Western countries we have acted too late and not with enough draconian fervor; the monster is out of the cage and will rampage for a year or more.
There are reasons to believe things are not so dire
The problem with all of the harrowing numbers cited: they all presume there is no latent immunity in the population.
(Note here that “immunity” is a probabilistic concept, I can be “immune” to a virus but if exposed to a large enough dose I will succumb.)
“70% of adults will get it” presumes a disease that has escaped containment and for which there is no latent immunity.
A 1% mortality rate suffers from the denominator effect of being measured against only those people exposed to the disease who actually can test positive. If you are immune to the disease (or sufficiently immune-hardened that it can not take hold) you will not test positive, and so won’t be counted in the denominator of that 1%.
Since South Korea and China are testing every close contact to a known carrier it seemed natural to assume that there was no denominator-error in their data — that of X people who accidentally ate a muffin with a speck of the virus on it at least 1% and maybe 5% will die.
But if there is a latent immune-hardened population the actual mortality-rate upon exposure will be proportionally reduced, perhaps meaningfully so.
How could there be a population with immune-protection already in place against a novel pathogen?
A substantial portion, perhaps 30%, of “common colds” each year come from a more innocuous form of coronavirus (of which there are 4); one reason why kids might not be so susceptible is because of frequent exposure to this form of the common-cold, in effect they have been partially vaccinated.
Of 24 healthy infants in the UK, every single subject had acquired antibodies against 1 of the 4 human coronaviruses before the age of 2 years.
Not only are children frequently exposed, but antibodies against one coronavirus seem to protect against another. Specifically, OC43 (green, above) is of the same type as both 229E and nCoV-19 (a betacoronavirus), and upon acquiring immunity to OC43, none of the children were subsequently infected with 229E. A similar pattern held for NL63 which appeared to confer protection against HKU1 (an alphacoronavirus).
It may that a recent prior infection with common-cold coronavirus protects against this recent scourge.
And there is reasonable evidence to suggest that there is a substantial immune-hardened population.
If you look at the contact-tracing studies, the disease is actually quite difficult to get from chance exposure, perhaps 1%.
Further, to explain the demographic skew of those that acquire the disease — even in clusters where all of the positive-testing cases are being found — almost requires some age-determined latent immunity.
Take the Beijing outbreak as dimensioned in the journal of infection:
If the amount of exposure to the virus mapped directly to the Beijing population demographics then the odds of developing an active case as a person under the age of 45 are 1/4th the odds as a person over the age of 65
Even if 0% of 65+ year olds are immune the Beijing data suggests that 75% of those under the age of 45 are.
A relatively simple way to test as to whether or not common-cold coronavirus conveys partial immunity would be to look at whether having school-age kids confers a reduced risk to the virus.
China has that data.
It’s possible that inoculation of vulnerable populations with common-cold betacoronavirus could be a mechanism to severely reduce global risks.
If you take those data and map them to global demographics you would infer that 55% of the global population is already effectively immune. Mapping to US demographics you would infer that 46% of the population is immune.
Further, if children serve as a source of immunity via common-cold coronavirus transmission, one would expect populations with a higher percentage of children to have a larger share of the population latent-immune. China’s one-child policy may have hurt it in this regard.
Combine these data with the quite-low transmission rate to close contacts and you can readily infer that the disease is actually quite manageable, so long as you actually attempt to manage it.
The problem is that the disease’s response to a policy change only occurs at a 2-week lag to the implementation of that policy change and so though Italy has already massively locked down we have not yet reached the point where new cases will actually begin to taper (and so aren’t quite certain in a Western country that acted late how high things will get before the tapering begins.)
In the US, where we have no idea how many cases there actually are, but we know that there are quite a few, and we know that those already in queue will impose a substantial and overwhelming burden on the hospital system, things can feel very scary indeed.
But given an expectation that there is actually quite substantial latent immunity, just shutting in older populations, going through a brief period of massive activity reduction, and rolling out testing and fever-checking regimes should be sufficient to bend the curve — as they did in China — even without taking the draconian actions that China required — separating parents from children etc.
A reminder: Wuhan is a city of 11 million and total infected there planed off at roughly 70,000. Your odds of getting the disease at all, in the worst spot in the world for it at the time, were roughly 1% (assuming you accommodate the policy-response.)
From a macro-economic and human perspective this implies that this event — these odd moments of self-quarantine, conducting zoom-conferences from our bedrooms — will prove a storm rather than a world-historical catastrophe.
It seems to me that people are considering whether we have suddenly found ourselves thrust into a world war against a piece of biological code, whether we will have to batten down and basically wait out a vaccination or a cure; upon reflection it will not be nearly so severe.
That the response to this disease has thus far been chaotic and ill-planned does not mean that it will remain so. Over the past few days the steps that people have taken will have already begun to bend the disease’s curve.
And things will feel worse — they will feel much worse — but the underlying prospects will be improving.
That it will seem to then get worse, will cause politicians to tighten the reins even more, but this will be to productive purpose. It will break the disease’s back.
In the aftermath there will be testing regimes, temperature checks, heightened vigilance, political recriminations, people will worry about another round of it in winter and that will dog equity markets, but it will not come back at nearly the same severity.
Of course it feels scary; of course. But there is no need to fear.
