# Understanding Herd Immunity from Vaccination - How much is enough?

Nurses   (26,550 Views 19 Replies 1,386 Words)
by SummitRN, BSN, RN (Member)

SummitRN has 5 years experience as a BSN, RN and specializes in ICU.

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Herd Immunity means lowering a population’s susceptibility to a disease to the point where a disease cannot remain endemic, introduced outbreaks fade without intervention, and the vulnerable are protected by the immunity of the herd. Let’s learn about how we actually achieve herd immunity with vaccination and “how much is enough?” We’ll examine the basis of epidemic theory, and examine Measles vaccination and the Measles current outbreak as an example. Fascinating stuff ahead!

Herd immunity, or community immunity, is a great term we hear bandied about lately. Herd Immunity means lowering a population's susceptibility to a disease to the point where a disease cannot remain endemic, introduced outbreaks fade without intervention, and the vulnerable are protected by the immunity of the herd. Let's learn about how we actually achieve herd immunity with vaccination and "how much is enough?" We'll examine the basis of epidemic theory, and examine Measles vaccination and the Measles current outbreak as an example. Fascinating stuff ahead!

We can't learn everything in school. Most nursing schools offer Community/Public Health, but the epidemiology covered is largely conceptual. Few nurses see the most basic equations that underpin it. What did your school teach?

Arithmophobes, don't fear, there is very little math here. I'll keep it elegantly simple, nothing but elementary school arithmetic. It is not possible to include all details and variables in the scope of this article.

## Let's talk about epidemic theory!

Some diseases are very contagious. Others are less easy to spread. How can we quantify the propensity of a disease to spread? We start simple with the Basic Reproductive Rate:

R0 = Number of secondary infections from one index case in a fully susceptible population

Some diseases are very contagious like Pertussis and Measles. Influenza has a R0 of 2- 3.

If we have an index case of measles in an unvaccinated population, the second generation will have 12-18 infections. The third generation could have 144-384 infections. This is keeping it very simple. I promised. No matrix calculus! You can see, the outbreak grows with each generation. If I want the outbreak to shrink, I need an Effective Reproductive Rate ® less than 1, where each generation produces fewer infections than it started with. There are several ways to lower R, ranging from vaccination to quarantine.

## Herd Immunity

Herd immunity, or community immunity, means we have an effective reproductive number less than 1 due to immunity in the population.

Q0 = Threshold of the population that has to be immune to achieve R<1
Q0 = (1- (1/R0))

Great video illustrating Herd Immunity

We can achieve herd immunity for measles somewhere in the range Q0=0.91-0.95 and the converse 1-Q0, or 1/R0, is the maximum susceptibility rate of a population to still have herd immunity (5-9% population susceptible). But 91-95% isn't the same as the necessary vaccination rate for herd immunity. Let me break it down. Q0 is the minimum immunity rate we need in a population to make outbreaks immediately self-limiting; the minimum vaccination rate required to achieve Q0 is higher than Q0. Consider that we don't try to vaccinate everyone and vaccination does not always result in immunity.

We want to know:

V0 = Minimum vaccination rate, of those we can vaccinate, to achieve herd immunity?

The extra variables we need to know:

P = percentage of the population we could be vaccinating
C = seroconversion rate, percent of those vaccinated who become immune

Therefor:

V0 = Q0/(C*P) = (R0-1)/(R0*C*P)

For our measles example, recommended vaccinations are occurring at ages 12-15 months, so we have roughly 2% of the population who are susceptible, vaccination not due, so P = 0.98 in this example. Our seroconversion rate © is 95-97% (we can make this 99% if we redose MMR at 4-6 years) so we will just assume C = 0.97 to avoid complexity in this example. When we do the math we see:

V0 = 0.964 - 0.993

## So That Is How Much?

99%? That is a really high percentage of people that need to be vaccinated on schedule. Luckily, more complex models show we don't quite need 99%, but it is still high, higher than Q0. It should now be obvious why exemptions are a huge problem. Medical exemptions are incredibly few, almost negligible in rate for infants and toddlers. But personal and religious exemptions create a problem. 4% exemptions and we have no hope of herd immunity. Colorado has an exemption rate over 18% right now for MMR. The more exemptions, the faster and wider an outbreak spreads. Worse yet, studies show vaccine refusers cluster... so in a community there is a very high R, creating a larger than expected infected population that then exposes the rest of the population. Remember, ~1-3% of the vaccinated are still susceptible to measles due to no serconversion or becoming immunocompromised.

Vaccine refusers who are misinformed or who don't understand the impacts on society are a major problem, but there is another flavor. Some people ascribe to the disproven theory that infants and toddlers cannot handle CDC recommended immunizations and they should be delayed. Dr. Bob Sears popularized this idea and invented an alternate schedule to go along with it. He recommends age 5 for vaccinations like MMR. What if everyone followed that? 7% of the population is under age 5. Calculate V0 for P=0.93 and we get impossible values. If everyone followed Dr. Sears vaccinate-at-entry-to-school philosophy, we would have a national immunity rate to Measles

Herd immunity could protect the vulnerable

With a highly contagious disease like measles, even in the best of circumstances, even with the highly effective vaccine, we operate in the grey zone maintaining of herd immunity which is why we still have to have some public health measures to combat new outbreaks. We are not in the best of circumstances thanks largely to anti-vaxxers. The US has a vaccination rate worse than Mexico. The US rate grazes the bottom end of the herd immunity range. We have to make up ground with public health intervention to stem outbreaks. CDC, Public Health Departments, and healthcare providers are educating the public, examining records, tracing contacts, isolating measles cases, and quarantining exposed susceptible people.

Over the edge of herd immunity - Needs a helping hand

Without these public health measures, measles would become endemic in the US again. It still could end up that way. Luckily, the Supreme Court ruled that the government has the right to mandate vaccines and enforce public health measures including quarantine.

Vaccination sometimes spawns talk about freedom and rights. The math above makes it clear that individual decisions to vaccinate affect all of society. Public health measures range from low key to draconian. Lower vaccination rates require more intrusive and draconian public health measures to stem the tide of an outbreak. How do you feel about this?

This Measles outbreak has spread to 18 states and Mexico with hundreds of people are quarantined or isolated. The outbreak would not be so big if more people vaccinated. The outbreak would be worse without the public health establishment fighting the outbreak. It is not over yet. It will happen again.

Nurses have many roles in this battle for public health, depending on our specialty. All of us can educate and advocate. What is your role?

References:

SummitRN's spouse had a reaction to Dtap and the doctor stopped the series, ultimately they caught pertussis and were sick for 5 months. SummitRN is not an epidemiologist, just a nerd, and accepts all the fury that any epidemiologist or public health expert feels is justified for oversimplification and omissions.

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Thank you for a well-written and clearly presented argument for enforcing vaccinations wherever and whenever possible. There are now so many people relying on the "rest of the herd" to protect them (while they refuse to vaccinate their own children) that there IS no "herd" and there's the outbreak...spreading widely and quickly

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heron has 40 years experience as a ASN, RN and specializes in Hospice.

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Thanks from me, too. The concept of herd immunity is the basis for mandatory vaccinations. So many of the arguments against vaccination seem to ignore the concept in favor of individual autonomy. I haven't yet read any anti-vax acknowledgement that they are using herd immunity to protect their own kids. And taxpayers' healthcare dollars to treat others' preventable infections/complications. They assume, of course, that when herd immunity is broken, it won't be their kids suffering those complications.

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Red Kryptonite has 3 years experience and specializes in hospice.

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They assume, of course, that when herd immunity is broken, it won't be their kids suffering those complications.

The latest one I've seen is when challenged after saying, "No one dies from measles!" with actual numbers of people who did die of measles, they come back with, "But that's in the 3rd world, not here." So there's classism and racism at play in this discussion too.

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macawake has 10 years experience.

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SummitRN, thank you so much for the effort you've put into writing this excellent post!

I hope it will have an impact on those who are unsure/on the fence regarding immunization vaccines and fingers crossed that it might get through to a few of those who've so far had their minds closed off to facts due to fear or a lack of scientific knowledge/understanding.

I find it so difficult that to grasp that so many people seem to shrug their shoulders at the very real threat that diseases like for example measles can actually pose. I think that because they've become so rare or near eradicated (thanks to immunization vaccines!), that many don't remember or realize the devastating havoc they can wreak on a population.

If vaccination rates decline the wake-up call will come soon enough

Edited by macawake

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. So many of the arguments against vaccination seem to ignore the concept in favor of individual autonomy. I haven't yet read any anti-vax acknowledgement that they are using herd immunity to protect their own kids.

Oh, I sure have. Along with the tone that implies "you all took the risk for your kids so I don't have to for mine". Have heard more than one actually state "that's what herd immunity is all about; I know my kids are protected already, so why put them at risk more by getting vaccinated?" How about because there are now enough people who think like you do that the rest of us who did the right thing can't protect your kids OR ours now, thanks for that.

Can't fix stupid.

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Ok, now THIS is special:

Apparently there's an idea breaking in California (shocking) that MEASLES PARTIES are a good idea. Yep, just like the Chicken Pox Parties of yesteryear, some idiots believe that if they give their kids an exposure in this fashion, it's a "natural" immunity that is somehow better than getting the vaccination (instead of the actual DISEASE).

Again....can't fix stupid.

Just wait until the first kid dies from having attended one of these 'parties'. What fun THAT will bring...

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LadyFree28 has 10 years experience as a BSN, RN and specializes in Pediatrics, Rehab, Trauma.

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This article is timely and sooo needed for many posters; however there are going to be some who staunchly have their belief systems even though hard science is at play here.

I fear it will take an epidemic and then having more federal oversight to have these endemics be eradicated.

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And then the very same people who are screaming about individual rights trumping the rights of the rest of the population (greater good) might just have to get a Quarantine notice slapped on their doors.

With an extreme rare exception, the only people we see who have personally suffered from Polio are elderly; most people DON'T see the disfgurements so therefore believe it isn't possible any longer.....and you can bet your butt it sure is. Our generation did not see Measles, so......guess it's gone!! I haven't seen it personally so it must not exist? It can't be a real threat? Argh.

We have the luxury of significant personal freedom in the US, which has unfortunately led to people having the freedom to do themselves (and the rest of us) in.

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LadyFree28 has 10 years experience as a BSN, RN and specializes in Pediatrics, Rehab, Trauma.

8,427 Posts; 75,622 Profile Views

And then the very same people who are screaming about individual rights trumping the rights of the rest of the population (greater good) might just have to get a Quarantine notice slapped on their doors.

With an extreme rare exception, the only people we see who have personally suffered from Polio are elderly; most people DON'T see the disfgurements so therefore believe it isn't possible any longer. Our generation did not see Measles, so......guess it's gone!! I haven't seen it personally so it must not exist? It can't be a real threat? Argh.

We have the luxury of significant personal freedom in the US, which has unfortunately led to people having the freedom to do themselves (and the rest of us) in.

What people don't realize is that the complications of measles, especially in children, which if they survive, end up with cerebral palsy, so those "special needs kids" that they see in their communities whose to say that it won't hit home or rather, THEY contributed to their "special" status?

All for the sake of "personal freedom?" how about those kids "freedoms"?

Along with freedoms, there are responsibilities.

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Farawyn has 25 years experience and specializes in A little bit of everything..

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What they all said. Great article. As a school nurse it is very much appreciated. The math did do me in, but I got the gist of it.