About Herd Immunity

Herd immunity, or community immunity, refers to the protection that a high percentage of vaccinated individuals provide to individuals who have never been vaccinated or in whom vaccinations have failed against the spread of infectious diseases. The protection is provided because the disease cannot find new hosts to spread to after a certain percentage of the population is vaccinated.

Here is a good way to think of this: immunized individuals provide a type of firewall, protecting the unvaccinated from the disease.  This may be easier to follow with some cartoons depicting the general idea.

Here are pictures:

= Immunized individuals

= Unimmunized individuals

=Infected individuals



In a community with many vaccinated individuals, the unvaccinated individual is protected against the spread of disease.



In a community where vaccinated individuals fall below a certain percentage, there is less protection for unvaccinated individuals who who will then catch the disease.


In a community where the majority of individuals are unvaccinated, only individuals resistant to disease or vaccinated will be protected from catching disease.



Why is the concept of herd immunity important to understand in the context of vaccinations?  Many vaccines are given at specific ages and children who are younger than that age are susceptible to disease.  In addition, there is a very small percentage of individuals that vaccines do not provide a protective effect for a variety of reasons.  These people often don’t know their routine vaccinations have failed but they and young children are protected when they’re surrounded by a community of individuals who have been vaccinated.  The vaccinated people provide a wall of protection.

In communities in which vaccination rates have decreased below the herd threshold, or the percentage of people that must be vaccinated to protect unvaccinated individuals, you have epidemics of measles, whooping cough and other diseases easily prevented by vaccination.  Often, in the case of these epidemic outbreaks, the individuals who become ill are both the unvaccinated children as well as children too young for vaccination.

In addition, the proper vaccination of certain groups of people can actually have a protective effect for groups susceptible to disease.  A recently published paper in Science magazine detailed that communities with high childhood vaccination rates for the H1N1 influenza vaccine actually had fewer elderly individuals die or become seriously ill from the disease.

I hope this brief overview of herd immunity was informative.  If I got something wrong or you have questions, feel free to let me know!  I also want to thank all the scientists reading Persephone who volunteered their brains for me to pick ““ I’m certainly going to take you up on it!

My next column will appear in two weeks.  Please send me questions or suggestions at, otherwise, you’re going to have to read about what I’m interested in.

Selected sources and interesting reading: – general information on vaccination – why vaccines work – this is an excellent resource available to everyone.  There are nice videos illustrating the concepts I explained above. – mythbusting misconceptions about vaccinations. ““ a section geared toward parents interested in learning about vaccination. – a decent overview of herd immunity.

Jason M. Glanz et al. (2009). Parental refusal of Pertussis vaccination is associated with increased risk of Pertussis infection in children. Pediatrics: Vol. 123 No. 6 June 2009, pp. 1446-1451.

Jon Cohen. (2004). Immunizing kids against flu may prevent deaths among the elderly. Science: 12 November 2004: 1123.

Joseph N. S. Eisenberg et al.(2009). Protecting the herd from H1N1. Science: 13 November 2009: 934.

Image Credit from Wikimedia Commons




19 replies on “About Herd Immunity”

Thank you so much for this article!

Autism is something I’m very passionate about, and it infuriates me to no end that people are willing to jump to the conclusion that vaccinations are linked to autism just because Jenny McCarthy said so and without doing any research of their own. Skipping a child’s vaccinations puts that child in danger for so many diseases that are no longer part of our lives–smallpox, polio, whooping cough, etc. People are putting their children’s lives and the lives of people who many not have been able to be vaccinated. Just because people are being irresponsible and not doing their own research does not mean that they have a right to put the rest of us in danger of some kind of outbreak.

I just registered so I can jump in on this conversation! What your article doesn’t mention is the fact that vaccines are not 100% effective, so there are many more holes in herd immunity than most people realize. But they’ve been studied and refined to a point where it’s statistically okay — as long as everyone gets vaccinated. The MMR vaccine is only 95-98% effective against measles, so one out of every 20 vaccinated people really isn’t vaccinated — but they ARE protected by herd immunity.

Also, boosters are absolutely necessary because they provoke the antibody response (formed — but weak — after the first vaccine) and make sure it is strong enough to respond in the event that the actual virus invades. Make sure you get your boosters on time! Go get a tetanus shot, if it’s been more than 8-10 years since your last one!

(I’m a vet tech so vaccines are a big part of my life!)

I alluded to the fact that vaccines weren’t 100% effective – I said that, in a certain percentage of individuals, vaccines failed for unknown reasons.

I did not mention boosters – thanks for bringing it up. In fact, in recent years, pertussis has had an outbreak among older individuals in which the original vaccine efficacy (vaccine strength) has waned.

Glad to have you registered! Thanks!

Oh this is such a hot button topic in many parenting communities. It never crossed my mind not to vaccinate. I was lamenting about my infant getting a shot once to my grandmother (who’s now 97), and she got incredibly serious.

She told me about her brother who died from diptheria, and about how scary it was when her sisters got the mumps (or the measles– or both) and how her family had to post signs on their front door warning people that the disease was in the house.

Fortunately, my kids never had anything more than the mildest of reactions (redness, low fever) to any of their shots.

I’m glad you noted that some people get vaccinated and it simply doesn’t work for some reason–the vaccine just doesn’t stimulate their immune system. I think a lot of people have no idea that is possible, or that those people have no choice but to depend on other people getting vaccinated to protect them from disease.

I have a friend (who happens to be an RN), who has been vaccinated against chicken pox SIX times–it’s a two-dose vaccine, and she’s gone through both rounds three separate times over a period of several years. She still has no immunity to chicken pox, and she never had it as a child. There’s no way to know why it’s not working for her; it’s just a rare case. But her health depends on other people being vaccinated, because if she gets chicken pox now, as an adult, it could be fatal.

Yep–she only found out it didn’t work because she works in health care. Since she hadn’t had chicken pox as a kid, she got vaccinated so she could work in a hospital, and had to prove it took with a titer test. Happily, she’s still allowed to work; she just had to agree that she was working with patients who might have chicken pox at her own risk.

This is a really big issue in my MPH program. There are a lot of mothers who are militant about vaccinations, but they can’t really explain as eloquently/simply why it’s important when speaking to other mothers. Unfortunately, giving everyone measles/mumps/whooping cough etc. do not sound as scary as the false link with Autism.

There is a basic fact that the people who erroneously believe that vaccines cause Autism tend to forget. The diseases that vaccines protect against kill children. That is why it is so important to be vaccinated. Autism, which, I reiterate, has been exhaustively shown to have no link to vaccinations, DOES NOT KILL CHILDREN.

ETA: Some of the links I provided very simply explain the importance of vaccination to parents. Perhaps point some of the people in your program in that direction.

This is an excellent explanation.

I just wish that it was easier to tell who had their vaccines and who hadn’t. We can’t all be hearts and stars.

A daycare center I was working with that cares for roughly 75 children (about 15 being under the age of 18 months) had a whopping cough outbreak two months ago. The parents had CLAIMED the child who introduced it was vaccinated against it but a notice from Health Services proved otherwise. After the notice was given to the center, EVERYONE including the pregnant manager had to take extreme caution. I actually called my doctor to confirm that I had been vaccinated since I would be at the center for a couple of days. The room that the infants were in had to be sanitized at least three times a day in case someone from another room spread it into that room because all the infants were too young for the vaccine. This incident was dangerous, careless on the parents’ part, and expensive. Thankfully no one died but the risk was real.

And while getting vaccines for your children is mostly a choice (I’m not certain on all state laws), you have to TELL people what’s up. Those hearts roaming around the stars aren’t always so obvious and they often need extra care and protection.

I know that many schools require proof of vaccination in order for children to be admitted. This includes some universities that now require immunization for meningitis prior to attendance. Unfortunately, it is legal for parents to choose not to vaccination their children. That seems like a very selfish and dangerous choice to me.

My university required proof and unfortunately all my original immunization records were lost in various bits of chaos. In order to even move into the dorms I had to see a nurse and either get new shots (omg, still sucks even when you’re older than 4) or base proof on prior history. Records aren’t concrete unfortunately. This is why I called my doc – I didn’t have any records myself. But I KNOW they’ve been done. My mom is all about vaccines.

In this case, the parents provided allegedly inaccurate records. (a polite and legal way to say – the documents may have not been what they seemed…)

Unfortunately whooping cough has at least two strains now that have mutated past what the DTaP vaccine can protect against. They probably mutated because of people not vaccinating their children, but now some vaccinated children can actually still get pertussis. I know in this situation that wasn’t the case, but I wanted to throw that out there. Vaccinate your kids, people! It might not always help, but it can provide partial protection!

(I’m in a WIC Emerging Infectious Diseases class and pertussis is the disease I chose to research and write about all term long.)

We live in a very educated but hippy liberal leaning neighborhood. Our mothers’ board rages with the vaccination debates occasionally. I don’t worry about my 2 year old catching something at daycare because she’s been fully vaccinated. What petrifies me is picking up/dropping her off at daycare with my 2 month old (who hasn’t been fully vaccinated yet). I have no idea what she’s being exposed to every time we go and get her sister…

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