The real story behind “Herd Immunity”

It’s not about stopping disease altogether. It’s about stopping outbreaks.

During the current outbreak of measles that seems to have originated in Disneyland, a lot of discussion took place in all sorts of fora about the science of immunization. One of the concepts that was explained time and again was “herd immunity.” (You may also have heard it being called “community immunity.”) Experts told us that high levels of immunization were needed to protect the “herd” (a group of people) from measles. But what does it all mean?


We’re Only Human, After All

Whether we like it or not, humans are animals. We like to live in groups. From families to neighborhoods to cities, we all cluster together for many different reasons. We do it for comfort, for safety, for mutual protection. Even the most stringent libertarians will be part of a group. Heck, even anarchists have a hierarchy to their organizations.

Organized, structured, hierarchical anarchy. (Image via Danny Howard on Flick.)

Because of our nature toward gathering in time and space, we also end up transmitting diseases to one another quite easily. Sexually transmitted infections are called that because they are transmitted via something that 99.999999999999% of the population does at one point or another in their lives. (Hint: It rhymes with “flex.”) Other diseases are transmitted by contaminated food and drink. Those things got contaminated because we humans clumped up in a spot and contaminated the water supply with our waste. Or we became too busy to grow and prepare our own food and someone with dirty habits did it for us.

Yet other diseases are transmitted by droplets in the air. It doesn’t matter how much you wash your hands, you’re going to catch those diseases if you’re not immune because you breathe the contaminated air. Measles is one such disease. Measles is so infectious that you have a very good chance of catching it if you go into a room where someone with measles exhaled and you’re not immune. I’m not joking. Measles is very, very infectious.


“The highly contagious virus is spread by coughing and sneezing, close personal contact or direct contact with infected nasal or throat secretions.

The virus remains active and contagious in the air or on infected surfaces for up to 2 hours. It can be transmitted by an infected person from 4 days prior to the onset of the rash to 4 days after the rash erupts.

Measles outbreaks can result in epidemics that cause many deaths, especially among young, malnourished children. In countries where measles has been largely eliminated, cases imported from other countries remain an important source of infection.” — World Health Organization


Good News, Everyone! We Have A Vaccine!

Immunizations are the best protection against measles because of our human nature. It’s hard to get someone with measles to a deserted island fast enough to prevent them from passing it on to someone else. It’s also hard to isolate them completely. By immunizing people, we guarantee (because the measles vaccine is highly effective) that the great majority of people will be immune against measles.

With enough people immunized, the virus has a hard time finding a suitable host to latch on to. You don’t have to worry about where people with measles were breathing. Where they at a church? No problem. Where they in a school? Who cares. If the “herd” is immunized at a high rate, you won’t have an outbreak.


This is not to say that you won’t have sporadic cases of the disease, even if you immunize 100% (aka “EVERYONE”) in the herd. Because we live in an imperfect world, the vaccine is not going to give immunity to everyone. The current measles vaccine — which is part of a combined vaccine called “MMR” — is 93% effective with one dose. Two doses give 97% effectiveness. So immunizing 100% of the population as currently recommended would only guarantee that 97% of them become immune under the best of circumstances. This leaves 3% of the immunized herd non-immune.

In the real world, more than 3% of any population will be non-immune because not everyone can get immunized. Those who are too old, too young, or too sick to be immunized are susceptible. Those who were immunized but lost their immunity because they’re on immunosuppressive therapy are susceptible. And those whose immunity waned for whatever other reason are susceptible. What is keeping them safe is herd immunity.

Circle the wagons!

When everyone who can be immunized is immunized, we are kind of “circling the wagons” around the susceptible people. Other terms used are “cocooning” or “enveloping.” That is, those of us who are immune serve as a line of protection to keep measles away from those who are not immune. But, again because we’re not in a perfect world, there will be some cases where measles gets through the barrier and infects someone.

When that happens, however, the virus has a hard time finding a second person to infect because so many people in the population are immune. This breaks the chain of transmission and stops an outbreak dead in its tracks. You would think that this is a good thing — and it is — but it is also the “bad” thing about herd immunity. The rest of the herd assumes that we’re all immune and gets worried when one or two sporadic cases occur.


What Do Jenny McCarthy & Friends Have To Say?

That a few cases still happen is the argument that anti-vaccine groups make against herd immunity. If there is one or two cases here and there, or even a small outbreak of three or four people, the anti-vaccine blogs light up with all sorts of accusations about herd immunity being a “lie.” Sure, they can’t come up with any alternative explanation as to why the incidence of diseases like measles drops precipitously after the vaccine was marketed, but they hold truth to their version of reality.

Jenny McCarthy, non-Immunologist

Here on Planet Earth, however, herd immunity is a reality, and it tells us how communicable diseases work. We have been able to determine, through epidemiology and some biostatistics, that we need a certain level of immunization in the community to prevent outbreaks. That level is based on the effectiveness of the immunization, the number of people who are immunized, and the virulence of the microbe we’re trying to avoid. Other variables such as the motility of the community (does everyone ride the metro?) and weather patterns may also come into play, but they are not used as often because they’re hard to measure and/or predict.


You Could Have Just Read This Conclusion

The gist of this post is that herd immunity is a real thing with real science behind it. However, there are those who confuse it with zero cases of a disease in the community. (The people aiming to misinform will say that cases of a disease within the herd shows that there is no such thing as herd immunity.) In fact, herd immunity just says that you won’t have outbreaks if you immunize enough people (or if enough people become immune the hard way by catching the disease). You’ll have a few sporadic cases here and there, but nothing like what we’re seeing now in the Disneyland measles outbreak. That outbreak shows how levels of immunization are low enough to allow measles to be sustainably transmitted from one person to another to another… Hopefully not toward a very susceptible group, like cancer patients and such.