I'd like to take you back to the '20s. The 1920s, that is. It was back in 1923, almost exactly 100 years ago that W.W.C. Topley and G.S. Wilson wrote the article in the 'Journal of Hygiene' in which they coined the term "herd immunity".
Today's blog is in answer to a question from Laura, who asked: This concept of "herd immunity," - what is it? Is it a just theory or is it a real, documented condition that can occur? How would it be achieved?
To answer that, let's get back to Topley & Wilson's article. Therein, they described how in groups of lab mice, experimental epidemics would naturally end - after enough of the mice got sick, that is. If you never introduced new, susceptible mice, then the epidemic would just, well, peter out. Of note: if new mice were added in to the population, the epidemic would, however, continue to wreak havoc on the poor rodents.
Herd immunity is simply what happens when an infectious agent, like a virus or a bacterium, can't spread throughout a population anymore because it keeps only encountering animals who have developed some immunity against it. And, as Sir Sheldon Dudley found out back in 1923, that concept can apply in humans as well as mice. Dudley was appointed to monitor diphtheria outbreaks that were occurring regularly at the Royal Hospital School of Greenwich. The school was made up of about a thousand young male students who entered in distinct batches a few times a year and slept in large dormitories. He found that "old boys", i.e. those whom had been at the school for a longer period of time, developed immunity to diphtheria during outbreaks, and were three times less likely to get sick during the next diphtheria outbreak as compared to the "new boys" who had just entered the school. However, it's worth nothing that while this "natural" way of achieving herd immunity - via people getting sick - did help the survivors from getting really sick a second time, it very clearly did not stop the responsible bacterium (Corynebacterium diphtheria for the nerdier of readers) in its tracks. In the 1930s, diphtheria was still the third leading cause of death in England and Wales. It was only upon the introduction of vaccination that cases started to drop considerably.
But let's jump forward to a more modern case. Not COVID-19, no. Let's take a look at measles. We have largely achieved herd immunity against measles - with the exception perhaps of some communities along the west coast of the USA and Canada where an insufficient amount of people were vaccinated and small pockets of measles broke out in the last decade. Because many people are vaccinated against measles, although the disease rears its ugly head on occasion, it doesn't spread like wildfire as it would in an unvaccinated population - that's why it didn't spread over the whole province of British Columbia, Canada, when some cases broke out in the city of Chilliwack a few years back. Because most people are vaccinated, it is relatively hard for the virus to 'find' a new, unvaccinated (read: unprotected) person to spread to. It works like this:
If few people are protected against an infectious agent, either by immunization or recovery, it can continue to find new people to infect (we can call them "hosts" for the infectious agent in question), as in the panel on the left. The dotted lines indicate how the infectious agent can jump from person to person. In the panel on the right, you can see that having a high amount of immune people doesn't necessary protect everybody from getting sick. You can see there is a patient zero who can spread to that one susceptible person nearby. But, having a high level of vaccinated people is a great way to protect that one other susceptible person - they effectively block off the route of the infectious agent to the person at risk.
Usually, scientists talk about herd immunity in the context of vaccination programmes, asking, "How much of the general population do we need to vaccinate, in order to protect the immunocompromised, unvaccinated, and "immunologically naive" (i.e. people who have never been vaccinated and also never gotten sick with the virus or bacterium in question)?" So, this is a question about simply reducing the number of potential "infectees" to a point below which the virus needs to really get spreading across the population.
To summarize so far: herd immunity is real, and it can be greatly enhanced, to the point of basically getting rid of the infectious agent in question, by vaccination.
However, it has recently become rather popular to talk about herd immunity in terms of "how many people need to get sick with COVID19 to stop the pandemic". That take is rather outdated, as it ignores that vaccines have been the key to achieving herd immunity for - as far as I know - every other contagious disease humanity has ever encountered. It is completely correct that herd immunity can be developed by "natural immunity" (i.e. the immunity that people develop after actually getting sick with the bug in question), as in the case of the diphtheria outbreaks at the Royal Hospital School of Greenwich in the 1920s. But, if we take that case, the herd immunity only protected the "old boys" from getting sick. In the real world, there are always "new boys" coming in (in the form of children being born, for example), and thereby a continual influx of potential hosts for the bug in question. The real world is not set up like the mouse experiments that W.W.C. Topley and G.S. Wilson performed in the 1920s - or rather, it is set up like their control experimental condition in which new, susceptible mice were constantly added, and the epidemic could go on forever. Unless, of course, we use the best tool that science has to give us to combat infectious disease: vaccines.
In other words, people getting sick and developing immunity alone has never, not once in the course of human history as far as I know, been sufficient to stop a virus in its tracks. So until next time, if you like the idea of herd immunity - get that sweet sweet vax!