Can someone explain "herd immunity" to the rest of us?

[milestogo]

Sounds like a risky bet from what I know.

[brokephibroke]

1. Simply put

The virus has nowhere to go.

[msongs]

2. hungry lion eats one zebra, decides to leave the rest alone til spike 2? nt

[OAITW r.2.0]

3. Risky at the individual level.

But I guess it means - after the carnage is burned through - those left standing will have developed a bio-defense against this specific virus. But what about the next one? Especially if you destroy the front line care-givers in this pandemic?

[herding cats]

4. The more people who contract the virus and survive, the more community immunity.

It disregards unnecessary suffering, death and ICU cases, but it's a basic premise.

[Horse with no Name]

5. Simply put

The reason this virus is so devastating is because nobody had immunity to it. Now that the virus has made its way through the masses, those left standing should have some immunity to it. The other thing that creates immunity is vaccine. That’s not available yet.
Once the herd (the public)!has a substantial number of infected and vaccinated people, then we should start getting a handle on it.

[herding cats]

9. Good explanation.

As you said, there's no vaccine. So, right now it's a toss of the dice if you survive the virus easily, or not at all.

[Horse with no Name]

11. Yes.

We need to start concentrating on IgM testing. That will show us who has immunity.
The rapid test is for IgG. That shows who currently has it.
The tricky part is that like H1N1, immunity isn’t a long lived thing. It will probably be necessary for people to get vaccinated annually.
But next year, even before the vaccine is ready, there should be some immunity and the virus won’t be as devastating to a percentage of the population.

[herding cats]

22. The one problem is this is a novel virus, and there's no hard data yet as to how long immunity lasts

We want it I be a year, which we all know doesn't even apply to seasonal flu. Which is 5 months, at best due to variations in stains. However, this isn't a flu and it doesn't appear to mutate as such, so maybe we could vaccinate once or twice a year (if immunity is short lived) and get back to normal once a vaccine is available. We just don't know enough to know yet, sadly.

We're all in this together. The majority of us will come out the other side. It's the rest of us I worry about.

[TreasonousBastard]

6. It's not a plan-- it's a result...

In a group, enough people (or cows, or hummingbirds...) have managed to become immune that the bug kinda wears itself out looking for new victims.

[milestogo]

8. Sounds like its a plan in Sweden.

[honest.abe]

7. The point when so many are immune it acts like a buffer to further spreading.

The problem is many have to die to reach that point.

[Amishman]

10. It means that enough people are immune that it can't spread well

Let's say the average person comes in contact with three people in a way that could inflect them. You get 3x exponential growth.

Now let's say 2 in 3 are now immune through vaccine or having been infected and recovered. Now only one additional person gets it. No growth and quickly dies out.

[C_U_L8R]

12. Everyone gets it. Some die. Some don't. Who knows.

But everyone gets back to work. And Republicans fantasize they'll inherit everything.

[Olafjoy]

13. IgM is the acute antibody, the first one made in the active infection.

IgG is the antibody produced next that stays around and indicates immunity and has memory and will signal the immune system to blast out antibodies if the antigen (virus) is encountered again.

[judeling]

14. It is not a dirty word.

We get Flu shots and vaccinate our kids to induce or maintain Herd immunity.

[Bernardo de La Paz]

15. It is the logistics curve. Think an exponential and a reverse exponential glued together

Basically it is exponentially increasing until 50% and then exponentially decreasing, which means a long taper as the human population gets saturated with infection. It describes in a way the population growth of antibodies in the human bio-culture. https://en.wikipedia.org/wiki/Logistic_function#In_ecology:_modeling_population_growth

What is the intuitive explanation? We all know that it seems like infections (unchecked) blow up in an area. This is because from this virus's point of view new hosts are easy to find and you're only killing off something from 1% to, say, 5% of hosts. But at the halfway point, infected people start to outnumber uninfected. It gets increasingly harder to find hosts. So the curve slows down and starts running out of steam. At some point like, say, 90% infected, what remains are pockets that don't communicate much with others. So the top flat part of the curve is never really 100%. This is effective herd immunity. The herd is essentially immune and its as if the infection has died out.

Now there are many confounding factors at play, but basically it is a logistic curve. For number of infections think of the vertical as 0.5 = 50%. Think of the horizontal as time. You can think of Month 0 as the worst month of the epidemic.

Logistic curve (Wikipedia):


The rate of change of the curve is the famous Curve We Are Flattening, because that is the rate at which we adding new cases. Technically it is called the logistic distribution (though it is not a probability distribution). https://en.wikipedia.org/wiki/Logistic_distribution As you lock down, you slow the rate of infection, which flattens the curve and spreads it out so that hospitals and health care workers are not overwhelmed.

The logistic distribution is the derivative (slope) of the logistic curve.
The logistic curve is the integral (area under) the logistic distribution.

There. You've just learned a bit of calculus. An example of the Fundamental Theorem of Calculus.

Logistic distribution (Wikipedia):

[LeftInTX]

18. Would the area under the magenta curve be the same as the blue curves?

(Can't tell with green and red because their integrals are not complete)

[Bernardo de La Paz]

19. Intuitively by eyeball, I agree. But I can't go now to web page & work with the math to know. nt

[qazplm135]

16. think of bell bottoms.

in the 70s, no one was immune from wearing them so everyone had them.

Now, most are immune so only a few hipsters contract the disease of wearing them.

[yewberry]

17. Herd immunity.

(I am not an epidemiologist-- I am someone interested in viruses and epidemiology.)
This is a new ("novel&quot virus. We haven't developed immunity to it because we haven't been exposed to it before. Individually, our bodies may be able to fight off the infection, but we appear to be able to carry and shed the virus whether we're symptomatic or not.

For a virus like measles, most of us have had a vaccination, so our immune systems have experience fighting it, and for viruses like the common cold, almost all of us have some degree of immunity because we've been exposed (repeatedly) to some strain and we don't pass it on. That's herd immunity. Once a large majority of people are immune (or even partially immune) they are passing the illness to fewer people. That's the key.

The R0 (R-naught) number is a basic reproduction number for a virus, and refers to the average number of people a single infected person will infect in a population that hasn't been exposed to that virus. That measurement is not an inherent character of the virus-- it changes based upon populations and mitigation efforts. For example, this virus in Seattle has a R0 of just over 1. Global R0 from WHO seems to be between 2 and 3.

Good description from The Atlantic:
https://www.theatlantic.com/science/archive/2020/01/how-fast-and-far-will-new-coronavirus-spread/605632/

[PoindexterOglethorpe]

20. Herd immunity really is a good thing.

When enough of the herd are immune, because they've had the disease before or have been vaccinated, a new outbreak of that disease can't gain much of a foothold, and so doesn't have a chance to infect very many before it dies out for lack of fresh blood, so to speak. Herd immunity especially protects those who are prone to a bad outcome, such as having a compromised immune system and so can't safely be vaccinated.

It's important to separate the overall goodness of herd immunity from how hard it's probably going to be to get true herd immunity from this virus. As others have already pointed out, it's a "novel" virus, meaning brand new to our species and so essentially no one has immunity to begin with.*

Several things are confounding. One is that a lot of people who contract this virus, maybe even a majority of them, have few or no symptoms. What's not clear so far is to what extent someone who has had this and gotten over it is now permanently immune, as is the normal case for a viral infection. There are some reports of people apparently getting it again, but it's possible they simply weren't fully over it and relapsed, rather than started an entire new round of Covid-19 infection. Right now we don't know.

Yet another issue is that so very many people have underlying health issues. Smoking, diabetes, obesity, most auto-immune diseases, currently undergoing chemotherapy, on long term immune-suppressant drugs because of organ transplant, and various chronic diseases. All those people are far more likely to get really, really sick and perhaps die if they get this. Or have an extremely long recovery process.

Back to herd immunity. If those who get this and recover are then permanently immune, the more people who get this (whether they recover or die), the better off the herd will be.** Or at least until a good vaccine is developed and distributed to all.

*I'm going to go out on a limb and suggest that there really may be rare individuals whose genetic/immune system make-up actually has them immune, but they are going to be a tiny fraction of a fraction of one percent, so won't have any real effect on the development of herd immunity here.

**Don't get me wrong. I'm not saying a lot of people should get this and die. I'm just looking at the herd as a whole.

[milestogo]

25. Thank you, that helps.

[SoCalDem]

26. without press coverage,most people would never know

the true carnage, since people die one by one..Historians do their work later, and we do find out, but like anything, its importance is lessened over time.

If I don't know you, and your grandma, grandpa and a few uncles and cousins die, it's of little daily consequence to me, but if it hits my family, it's a big deal..

Herd immunity is republican by nature

[uponit7771]

21. The problem with herd immunity is 60% of the herd could die and the rest MIGHT become immune. In the

... case of CV19 it looks like 20% will be hospitalized.

screw that, that number is way to high to screw around with who can die

[backscatter712]

23. After everyone in an area is immunized (the easy way or the hard way), the bug runs out of new hosts

The sane policy would be to develop a vaccine, then when enough of the population is vaccinated, if someone catches the bug, there aren't enough opportunities for it to move to new hosts before the first host dies or fights off the infection.

In the absence of a vaccine, Republicans have suggested giving the population herd immunity the hard way - by letting everyone catch Captain Trumps. It's the Lord Farquaad approach.

[dalton99a]

24. From Wikipedia:

[denem]

27. Culling sickly animals.

(Boris-Swedish model)

[backscatter712]

28. Boris almost culled himself.

There's an own-goal for you.