Herd immunity and isolation

The Quarantine Zone
1

Okay, again, I’m not a medical talking guy at all, so please use small words and correct me if any of my premises are wrong.

So, my understanding is that for this whole thing to play out, we have to have “herd immunity” which my understanding is that enough people get infected, survive (or not) and then are immune (at least for a period of time) so that the virus eventually dies out because it either kills the infected person, or the infected person kills it, and there are not enough other non-immune people to spread to which allows its survival.

So does isolation stop this? For simplicity, let’s say that we have a community in West Texas (just to make it up). There are 15,000 people there. When all of this started, they built an alligator filled moat around their community with fences and rifleman posted at the top of the fence with enforceable shoot to kill orders for anyone trying to come in. Let’s not fight the hypo. Nobody gets in or out of this community.

During the isolation, they determine that nobody in the community has Covid-19. When they end their isolation in a month, six months, a year, five years, will they be in danger of contracting the disease as they have no immunity to it?

I guess a related question, and one that sparked this thread was the Spanish explorers exposing the native populations to smallpox. Were they Spanish infected with smallpox or were they somehow carriers with no symptoms? Many thanks.

2

The members of the hypothetical communit will be in danger of infection if they encounter an infectious person. And, if one becomes infected, it will rip through their community, since they have no herd immunity.

The idea with herd immunity is that those who are susceptible to infection (1) are less likely to become infected , since since they will not meet many infected people, and (2) if they do become infected, are unlikely to pass on the infection to many others, since they are unlikely to meet many people susceptible to infection. Your hypothetical community will benefit to some extent from factor (1), but not at all from factor (2).

3

I have been wondering about mutations.
This virus is highly infectious. If we went for herd immunity that would mean a very high number of infections. Would that also increase the number of successful mutations? Then looping around to infect those that had developed an immunity to another mutation? I suppose that different viruses have differing abilities to mutate. Our yearly flu seems to mutate quite easily and require different vaccines each year.

In some case. Might trying for herd immunity just keep multiplying the virus?

4

Let’s start with clarifying what herd immunity is.

Herd immunity is when spread decreases because enough of the exposed population are at least relative dead ends to further transmission that new infections decrease.

Resolving from an infection is one potential way to have enough immunity to be that relative dead end. Vaccination is usually a safer way, albeit not usually as effective of one. Some could be dead ends because of something else about them (e.g. other sorts of infections, or vaccinations, in the past that keep them from becoming as contagious after exposure, in a non-specific way). And of course if a specific circumstance has fewer exposed, then the fraction needed to be relative dead ends is less. More exposed? Then the fraction functioning as dead ends if exposed has to be higher.

So far so good?

Does not mean the germ fails to survive, just not additionally spread.

Does not require that every relative dead end from exposure is a 100% dead end to further spread, and in fact usually does not. I don’t know of any immunization that makes each person vaccinated 100% protected if exposed, for example, but a big enough fraction immunized with something that decreases transmission enough, still does it.

The issue with herd immunity as the end game is how it is achieved
So that as preamble let’s talk about your hypothetical isolated town.
Yes, unless something else has changed by the time they open up they are in the same place as they were before they closed off.
What can change?

One possibility is staying completely isolated until a safe and effective vaccine that can achieve herd immunity by that means is available, assuming that such some time occurs. Or until at least much more effective treatments are developed (and this requires more than a couple days shorter of hospitalizations as a meaningful outcome).

Alternatively they can better prepare for the infection to come through their town by doing any or all of the following -

They can increase their capacity to deal with the number of infections so their system does not become overwhelmed.

They can institute dynamically changing societal policies and procedures that keep spread of the disease at a rate and time of year that does not overwhelm their systems. (You really do not want a surge synchronized with influenza.)

They can develop policies and procedures that best protect the most vulnerable the most (at the least harms to them) so that herd immunity (if achieved) by infection causes the least morbidity and mortality possible.
FWIW the hypothetical is real world. China has 1.4 billion people and reports only 0.006% of thier population as infected. Okay we suspect they’ve under-reported, but even if the number is off by three orders of magnitude, it would remain that they have a small fraction infected …achieved by ongoing restrictions, both internally and for international travel preventing imported cases, that few other societies could implement for any length of time, let alone in a prolonged manner.

Other nations have achieved similar low rates by effectively isolating themselves from new cases coming in with limited travel, strict quarantine of those coming in, and rigorous contact tracing of any cases that slip in despite those measures. Not quite alligator moats but effectively the same end.

They are in circumstances analogous to your hypothetical. Their plan for now seems to be to maintain their isolation/control measures hoping for things to change, for however long that takes, and the change they seem to be banking on is a safe and effective vaccine.

5

It’s not necessarily based on the number of non-immune people. It’s how many non-immune people do each of the infected infect. That’s the R[sub]0[/sub] value.

We don’t have to reach the point where everyone has had the disease to beat this. R[sub]0[/sub] values are not embedded into the virus itself, they are based on behaviors, populations, etc. The most infectious disease ever known has an R[sub]0[/sub] of 0 if the sole person who has it is quarantined until they either beat it or succumb to it. If we are able to ramp up testing during this isolation, to the point that we can positively identify everyone who is currently infected, then do quarantining (and contact tracing since we aren’t standing guard around those quarantined) of the infected, we can reduce the R-naught to below 1 pretty easily. The current hurdle seems to be the first word of that last sentence.

6

Yes, in terms of herd immunity, isolating every single person who is infected, including the majority who are asymptomatic, presymptomatic, and minimally symptomatic, before they infect anyone else, in an ongoing manner from here forward, until and if herd immunity is achieved by way of mass immunization, inclusive of all imported cases as travel is allowed, would reduce in a specific circumstance with very few new exposures, and even none in the “resolved” bucket could be enough to have R effective below 1.

And certainly such has been combined with relative isolation from imported cases, and sometimes more broadly implemented societal measures, to prevent individual countries and regions from developing sustained community spread.

For the hypothetical discussion let us assume that such (inclusive of preventing new imported cases from spreading) is possible within any given community. Yes a very big IF, and very open for debate as to whether or not it is possible in the U.S. at any cost, let alone at what costs, but no fighting the hypothetical. (It would be a hijack of this thread if nothing else.) It might still be best to consider that in the bucket with the alligator filled moat, in fact to no small degree cannot work without a metaphorical one. It works while in place but once released one is in the same place unless something has changed in the interim. One must maintain that way forever (or until a safe and effective vaccine is widely available and distributed, whichever comes first) or create other changes.

7

That is a gross misunderstanding of how all of this works and even if herd immunity is necessary. It doesn’t require mass immunization (although that can certainly do the trick), it requires an R[sub]0[/sub] of less than 1. If 100 people are infected and our behavior is such that we have an r-naught of .5, then they will infect 50, who will infect 25, who will infect 12, who will infect 6, who will infect 3, then 1.5, then we’re done. That can be reached through behavioral changes (such as social distancing), testing, quarantining, contact tracing, etc. If we are able to lower it below 1, we win, with or without immunization. Could it flare up in some areas due to not following proper guidelines? Sure, but repeating the above get it back under 1. Could it come back with a vengeance in the future if we don’t come up with a vaccine? Absolutely, but again, we know how to reduce the value. Herd immunity is certainly one way of dealing with something like this, but if it requires every get it and hope they remain immune until a vaccine comes along, I’d rather work on changing the r-naught instead.

Is your argument that we should just say fuck it and let everyone get it and see who survives? If not, what exactly do you propose we do?

8

Hmm.

I could have sworn someone just claimed

In point of fact your last post is a gross misunderstanding, because I was agreeing with you that such is one approach, without engaging in any debate as to whether or not it was possible or practical to achieve. Yes, maintaining R effective less than one by some combination of keeping new cases from being imported in (isolation to some degree, the op’s premise), effective contact tracing and strict quarantines of all who are infected (be they the majority asymptomatic, presymptomatic, mildly symptomatic, or sicker, and their contacts for 14 days), and global social distancing, for some indeterminate open ended period of time, is an option, for so long as it is maintained.

That’s the op. Options that aim to keep new infections at near zero, be it the moat, or some combination inclusive of massive testing, quarantines of all infected and those exposed, and continued restrictions on human interactions, like funerals, and dates, and weddings, and work, can work so long as they are continued. Once released if nothing has changed, then nothing has changed.
But since you are asking - I do not think such a plan as an open-ended future is possible or practical and believe it would cause huge harms in terms of deaths and decreased quality of life across the globe, and am a supporter of the alternative as listed already but sure I’ll repeat:
Increase capacity to deal with the number of infections so the system does not become overwhelmed.

Institute dynamically changing societal policies and procedures that keep spread of the disease at a rate and time of year that does not overwhelm the systems. (You really do not want a surge synchronized with influenza.) We can learn from the experiments going on across the world as various countries and regions approach beginning relaxation of restrictions in different ways, and become informed by more real data as it is collected. Key information is still lacking. Part of this is learning which restrictions give the biggest bang for the buck, so to speak, and which cause much harm but little gain. As of yet we do not know.

Develop policies and procedures that best protect the most vulnerable the most (at the least harms to them) so that herd immunity (if achieved) by infection causes the least morbidity and mortality possible. We have globally failed there so far horribly, and not even considered the harms side.

Along the way restrictions get lightened without overwhelming systems. If a vaccine comes, wonderful. If not then the careful process, protecting the most vulnerable, gets there eventually in a way that minimizes the harms.