Three Things to Know about Herd Immunity by Dr. David Pate

What is herd immunity?

The concept of herd immunity is used in the context of contagious, infectious diseases (most often viruses) and usually vaccine-induced immunity. The idea is that there are some in our population who may not be able to be vaccinated (e.g., newborns or those who have significant immunocompromise, especially when all we have is a live virus vaccine), those who can be vaccinated but may not develop a robust immune response (in some cases due to limitations of the available vaccine and in some cases the elderly who may not generate the same level of immune response as someone younger) and those who simply refuse to get vaccinated.

Herd immunity describes the point at which enough people in that population are immune (it could be through natural infection, but more often as a consequence of vaccination) that the virus cannot efficiently transmit within that population, so while a single of small group of people may be able to be infected for any of the reasons I stated above, generally you don’t see large outbreaks as we currently are for COVID.

Herd immunity helps protect those who remain susceptible to infection in the population because the large number of immune persons means that these vulnerable individuals are far less likely to be exposed to someone who can transmit the virus to them.

What determines the level of immunity a population needs to have so that we have herd immunity?

It is important to understand a few principles. First, the more contagious a virus is, the higher the level of immunity required in a population to provide herd immunity. For example, measles is one of the most contagious viruses. To avoid outbreaks of measles, we believe at least 94% of the population needs to have immunity.

Second, this was much simpler before we became a global society. Just to illustrate the point, let me make up a scenario. So, my best executive assistant ever just had a baby this month. Let’s say that in Boise, we have great herd immunity for measles. But, let’s say that she decides to go to a resort town in Idaho for the holidays later this year to be with family and to go out shopping for gifts in this resort town. However, this resort town is comprised of antivaccers, and the level of immunity is only in the low eighties. There are other visitors to this resort town and one family brings a child who develops measles while visiting. Measles begins to spread like wildfire (just to put things into perspective, measles is probably about 6 – 7 times more contagious than COVID). While my assistant’s child was not yet at the age to be vaccinated and thus remains susceptible, but was reasonably well protected in Boise, this beautiful little girl is now highly vulnerable in this community they have traveled to. So, when we speak of herd immunity, we are talking about the percentage of immunity within populations, but it is important to know that the virus can continue to spread in populations that have not achieved herd immunity, and susceptible persons may benefit from the protections of herd immunity in one population, but if they move or travel to an area with a population without herd immunity, they will have to take many more precautions since they will no longer have the benefit of protection within the herd.

Third, even within a population with herd immunity, we have examples such as religious and communal living communities within those populations with herd immunity that did not believe in vaccinations and were relatively isolated anyway because those communities were closed and not being exposed to the contagion, but later had significant outbreaks when persons form those communities made an international trip and then returned to their communal setting, bringing the infection with them.

What is the level of immunity in the population needed to protect the vulnerable from COVID?

Short answer, we don’t know. I have seen or heard projections anywhere from 10 – 90%. That doesn’t help us much.

So, most of us have most commonly heard the projection of 60% of the population with immunity to protect the vulnerable from continued outbreaks of COVID. Where did that number come from?

Well, there are mathematical models to predict the levels of immunity required for herd immunity. The simplest is the equation 1 – 1/R₀, where R₀ (R naught) is the number of people that an infected person will infect when at the beginning of an outbreak, at which time no mitigation strategies have yet been implemented. That number for SARS-CoV-2 is believed to be between 2.2 and 2.7. So, if we plug in R₀ = 2.5, we get 1 – 1/2.5 = 0.6, i.e., the projection that herd immunity will require 60% of the population to be immune.

Now, while this is a convenient little equation, you may not be surprised to know that life is more complicated than this and that while this mathematical projection may be directionally correct, as I alluded to above, there are many factors that may contribute to what level of immunity will be required for herd immunity in different settings. For example, differing susceptibility levels in different populations could alter the level of immunity required for herd immunity. If young people are less susceptible to infection, a developing country with a much lower average age might require a lower level of immunity for herd immunity than a more advanced nation with a higher life expectancy. Social interactions can also impact this. In populations living in high density housing, significant crowding, or where people must show up to work sick in order to avoid losing employment, then higher levels might be necessary.

So, let’s go back to our equation, realizing it is not precise, it may not project the level of immunity required for populations that vary significantly from others in terms of their susceptibility, social interactions and risks for exposure, but with that said, let’s see if we can make sense of the range of 10 -90% that you can find out there, depending on whose projections you are taking.

One more thing about R₀ before we begin. If a person who is infected infects less than one person on average, the virus is not transmitting efficiently and is unlikely to produce outbreaks and community spread of the infection. In other words, with an R₀ of <1, we don’t have a lot of concern about the need for herd immunity. So, if R₀ was 1.1 for this virus (which I don’t know anyone who believes that), then the level of immunity for herd immunity would be projected to be 1 – 1/1.1 = 0.1 or 10%. This is why I don’t currently believe that these low-end projections for herd immunity for this virus of 10 – 20%. In addition, while we still don’t know the percentage of Americans who have been infected, if you assume that 40% of all the infections have been asymptomatic (this is a reasonable assumption) and if you look at the confirmed cases of COVID in the country and if you assume that everyone who was infected is now immune (another fact that we doubt is the case), you can end up with an estimate that about 10.8% of the US population has been infected so far. Obviously, we have not achieved herd immunity as COVID cases are soaring around many parts of our country, so projections as low as 10% certainly don’t seem likely.

If you use the range that is commonly believed to be the R₀ for this virus of 2.2 to 2.7, then you get a range of projections for the percent of the population with immunity needed to get herd immunity of 0.55 – 0.63, or 55 – 63%. That seems right to me, but as I said, we just don’t know. To the extent that these numbers are somewhere close to the levels of immunity required for herd immunity, we are a long ways off from achieving herd immunity, and given the impact on our economy, health care costs, the burdens on hospitals, the complications that some people appear to be developing post-COVID and the lives lost with just getting to 10 – 11% of Americans infected, there simply isn’t a reasonable basis to support the lock granny up, throw caution to the wind and get back to life as normal proposals to accelerate us getting to herd immunity, if that is even possible.

What do I mean, “if that is even possible?”

Well, herd immunity is based upon durable immunity in the herd. We still don’t know whether people who have been infected with SARS-CoV-2 are immune, and if so, for how long? There has been a lot of talk about antibodies, as if that was the entire basis for immunity (which it is not). But, studies on patients who have recovered from COVID are not particularly reassuring regarding the antibody response. Studies have suggested that not everyone makes antibodies following infection, oftentimes the antibodies produced are not the desirable types of antibodies that we would guess would be protective, even those who do make these best kinds of antibodies often do not produce them at high levels and a significant proportion of those who do develop antibodies experience a significant decline in their antibody levels in as little as two to three months. If antibodies are critical to the immune response against this virus (and I am not sure they are), and if these studies have accurately described the antibody response, then we would likely never have herd immunity through natural infection.

Now, don’t get too discouraged. First of all, our experience, though early, is that we have not seen a lot of cases where we believe a person previously infected with COVID has become re-infected. I don’t rule this possibility out, but if it does occur, it does not seem to be common. And, it may be because we have been chasing the wrong thing – antibodies. Oftentimes, with viruses, we find that the cellular immune response (see my prior blog post about this, as well as a soon upcoming one) is actually the most important. It may be that there are much higher levels of immune protection in our population than we think, since tests for cellular immunity are far more complicated, less available and not able to be performed in other than highly specialized laboratories. Regardless of this, it is very possible that the vaccines will stimulate a strong cellular immune response and a better antibody response than natural infection does, and in fact, with two vaccines for which we have data from early trials, that appears likely to be the case. The next challenge – get enough people vaccinated when a safe and effective vaccine is available that we can get somewhere around 60% of our population with durable immunity to this virus.