COVID-19 Dispatch #8: Let’s talk about Herd Immunity

by Frances Veasey, BS Biology, MS Biohazardous Threat Agents and Emerging Infectious Disease from Georgetown University

It’s week one million of the Great Distancing and everyone is getting antsy. People are looking for ways to continue life as it was before and they keep settling on Herd Immunity as the answer. Some say we should release everyone and get to herd immunity as soon as possible. Some say California already has herd immunity! (they don’t). So let’s talk about herd immunity and what it takes to get there.

WHAT IS HERD IMMUNITY?

Herd immunity is a state where enough of a population has acquired immunity to an infectious disease that it no longer will spread extensively. Vulnerable (non-immune) people are the fuel for the disease’s fire; its life source. For a virus to spread, it needs to move from an infected person to a vulnerable person. If it can’t find another vulnerable person before the infected person recovers (or dies) and is no longer contagious, that is a “dead-end host” and the virus’s transmission stops there. The more this occurs, the less likely the virus is to spread across a population. A population can be said to have herd immunity if enough people are immune that any outbreak will sputter out by itself for lack of good hosts.

To think of how this works: we are currently trying to extinguish the outbreak through extensive social distancing, in which we stop transmission by denying the virus access to new hosts by limiting how many people total the virus can contact. Herd immunity would deny the virus access to new hosts by ensuring many (if not all) people an infectious person contacts are immune. You can think of herd immunity as a “threshold” – if only a few people are immune, the virus can still spread pretty easily. There needs to be so few vulnerable people, a person is unlikely to encounter (and infect) any when infectious.

HOW MANY IMMUNE PEOPLE IS “ENOUGH”?

That is a good and important question, and there is no absolute number. The more contagious a disease is, the more people have to have immunity to get the herd effect. For example, measles is a very contagious disease and requires a very high threshold to get to herd immunity – around 93-95%. Polio is less contagious, but still requires 80-86% to be immune to trigger the herd immunity effect.

HOW DO WE GET HERD IMMUNITY?

There are two ways. 1) Most of a population has been exposed to a disease, and either recovered or died from it. Those who have recovered are immune, so most of the remaining population is immune. 2) An effective vaccine is administered to enough people that most of the population is immune.

It should be noted that we generally think of herd immunity as being achieved through vaccination. Herd immunity is a specific kind of protection: the kind that literally ends epidemics or outbreaks. So if you think about smallpox, for example – Europeans had a good amount of immunity to it because almost all were exposed to it as children and either died or survived; those who survived had lifelong immunity. But this was not herd immunity. If it had been herd immunity, the virus would have been extinguished. It is actually pretty difficult to get to herd immunity just through natural exposure, except in isolated populations where there is low risk of re-introduction. Herd immunity for smallpox was achieved with coordinated global vaccination campaigns; and what we think of as herd immunity for measles and other childhood diseases exists with assistance from epidemiology, testing, and public health measures that allow us to detect and isolate outbreaks before they become epidemics.

IN THE ABSENCE OF A VACCINE, WHAT WOULD IT TAKE TO GET HERD IMMUNITY FOR COVID-19?

First, in order to get a herd immunity effect, people who recover from COVID-19 will need to have lasting immunity. We don’t know if this is the case right now. There has been some data that shows people who had cleared the disease were later re-infected (caveat: it could be the same infection, not a true subsequent infection). The other possibility is that the virus mutates quickly/regularly enough that immunity to the strain you were infected with doesn’t give you immunity to strains that circulate later. This is why getting the flu one year doesn’t protect you from getting it the next year.

Second, there would be considerable collateral damage to embracing this strategy. Although we aren’t sure what the “threshold” is, a reasonable guess could be 70%. 70% of the US population (~330 million) is 231 million, so we will need about that many people to be infected and recovered in order to achieve herd immunity. Unfortunately, infections are not harmless. Again, we don’t have definite case fatality rates but 1% seems like a reasonable estimate based on what we’ve seen. This herd immunity strategy would likely result in 2.3 million U.S. deaths, in addition to suffering, massive medical costs, and potentially permanent lung damage in a large number of those infected. A friend of mine who knows economics estimated conservatively that one life is worth $4 million to the economy ($7-8 million is more likely). Even with the conservative estimate, that translates to over $9 trillion just in lives lost (not including medical care and other associated costs). So the cost isn’t even a trade-off between lives and economy – they are linked. 

WHAT IF HERD IMMUNITY IS THE ONLY WAY?

Let’s assume we have no other countermeasures available and our best shot at moving on with our lives is to achieve herd immunity. In this case, you could consider two options. One is to give up early and try to get to herd immunity as quickly as possible. This would mean dropping all the social distancing measures and letting the chips fall where they may. The other is to realize we probably will have to get to herd immunity at some point, but to try to go as slowly as possible. Of course, these are on a continuum with an array of in-between measures, but conceptually comparing the two extremes is most helpful.

Embracing the rush-to-herd-immunity approach is likely to cause death rates to soar as the hospitals are overwhelmed and can’t provide adequate medical services to COVID victims or patients with other diseases. In this case we could conceivably reach death rates of 10% or more, making our 2.3 million-death estimate look conservative. Are we prepared for 10 million deaths in the US? That’s a 5% case fatality rate, which is possible under these uncontrolled conditions. As the epidemic really gains hold, people will probably continue to distance themselves (or start if they hadn’t already). Economic hardship will befall us, leading to recession or even depression despite our commitment to keeping normal economic activities open as part of this strategy.

On the other hand, a slow burn is difficult in different ways. We are more likely to keep the death rate as low as possible (perhaps even below the 2 million estimate), but it will be exhausting. It might take 1-2 years to get 70% of the population infected. Can we maintain social distancing measures for that long? It’s already been a grueling slog. People have lost jobs; working parents are losing their minds trying to telecommute with kids in the house; children are being set behind in school; etc. It’s hard to see how this is a sustainable course of action either. 

ACKNOWLEDGING TRADE-OFFS

This thought experiment helps us understand the limits of herd immunity. Seeking herd immunity as a “control” strategy is an oxymoron in the absence of an effective vaccine – it’s essentially giving up and saying control is impossible. While we may achieve some form of widespread immunity over time, it’s in no way assured or even the best course of action. There are trade-offs to every decision, but probably none as dire or irreversible as the rush-to-herd-immunity scenario. In fact, each day brings us more information about controlling the disease; each month brings us closer to an effective vaccine. There may be breakthroughs in the next six months that allow us to control disease spread without sacrificing millions of our population (even more on a global scale – 55 million extending the estimates above). How tragic would it be if we choose to let millions of people die only to have a vaccine available months later?

It is tempting to look to a single, “silver bullet” solution to get us out of this difficult situation and place our hopes there. In reality, we have a lot of different tools at our disposal. We are not powerless against infectious diseases! We can save lives if we learn and act deliberately, acknowledging that there are no easy answers. We will all need to sacrifice in some way – and support those who are most affected. We’re in this together. 

Frances