The Facts About Herd Immunity and COVID-19

In this year of unexpected nonsense, the one thing that I would’ve never predicted is the hydra of herd immunity. When I was first asked by a journalist what I thought of herd immunity as an exit strategy for the pandemic, way back in April, I laughed nervously and said “No one is actually suggesting that are they? It’s absurd”.

Reader, not only were people suggesting it then, they’ve continued to do so almost every day since.

So, let’s talk herd immunity, what it is and isn’t, and why it still isn’t a solution to COVID-19 no matter what has been reported.

Herd Immunity

I’ve written about herd — or population — immunity a number of times before, but very simply it is traditionally the idea that if enough people are immune to a disease, they will prevent the people who are not immune from getting the infection. Mathematically, we can use this idea to calculate a herd immunity threshold, which is the proportion of people who need to be infected in a population before a disease stops spreading. For COVID-19, a very basic calculation famously puts the figure at about 70%, so most people would need to be infected before the epidemic would die out naturally of its own accord.

Now, people have proposed that actually, this basic calculation is not quite right. And it is not unlikely that they have a point — the threshold for a disease to stop spreading is probably impacted by a number of things including how we all behave, and the true figure could lie somewhat below 70% (or potentially above). However, despite this, it still makes no sense at all to talk about eventual herd immunity to COVID-19 for a number of reasons.

The main problem is something very basic — herd immunity requires IMMUNITY to the disease. When people are proposing herd immunity as an exit strategy for COVID-19, what they are implicitly arguing is that, once infected, you cannot get the disease again — you are immune.

Unfortunately, we know that this simply isn’t the case. There are already widespread reports of people getting reinfected with COVID-19, and worryingly some of these people are having MORE severe infections the second time around. This makes herd immunity in the traditional sense largely unreachable, because some people can clearly get infected and transmit the virus on to others over and over again.

We also don’t know how long the immunity will last even in people who get infected and are then immune. Some people may be immune for months, some for years, some for their entire lives — we simply have very little idea and won’t know for sure for a while yet. If large swathes of the population are infected this year but do not develop long-lasting immunity, chances are we’ll have epidemics in the future as well.

In other words, herd immunity is a largely impossible goal to reach for COVID-19 if we use the term in the traditional sense. Until we have a vaccine, it’s just total nonsense to suggest that herd immunity is a likely scenario for us to see.

So why are scientists meeting with the White House to discuss a herd immunity plan for COVID-19? The answer is pretty simple — they have simply repurposed the word. In fact, they are talking about a slightly different epidemiological concept — endemicity.

Endemic Disease

What is happening in the news today appears to be a very simple switching of the terminology. Rather than herd immunity in the traditional sense — remember, enough people protected from a disease to stop it from spreading — we are talking about endemic disease, where the infection dies down but then re-emerges on a regular basis.

A classic example of this is measles. Before vaccination, measles had a beautifully consistent biennial pattern — you’d see a huge epidemic one year, none the next year, then another huge epidemic the year after.

This sort of endemic disease is pretty common for respiratory viruses like COVID-19. We see similar patterns for influenza, for example, which flares up once a year then dies down over summer in most places. The thing is, endemicity has a huge issue when it comes to pandemic disease — unlike the traditional idea of herd immunity, if you’re looking at an endemic disease like COVID-19 most people will eventually get infected.

This is simply a function of how the infection dynamics here work. If the disease is circulating in the population, even if it dies down at times, there will be periodic outbreaks. If we were to, say, protect elderly people by limiting their daily contact to just one or two individuals, unless we did this for the rest of their lives they’d be at risk every time there was an outbreak. If we didn’t maintain these quite onerous restrictions on the elderly forever, they’d simply get COVID-19 the next time an outbreak swept through the population.

What we’d have, essentially, is what we had with measles before the vaccine was introduced. Pretty much every child was infected when they were young, either in one epidemic or the next, with only a scant few lucky children missing out. Since the disease came back again every 2 years, the only way to really avoid it was to avoid people altogether until everyone you knew had been infected.

Except, unlike with measles, you can get COVID-19 more than once. So not only will we have to deal with periodic outbreaks of some kind, we will also have to deal with some proportion of people who have had the disease previously getting sick a second time. This makes plans to protect the more vulnerable populations even more problematic —even if you staffed an aged care centre only with people who’d had COVID-19 before, you would expect some proportion of them to get sick the next time an epidemic broke out anyway, potentially infecting the vulnerable older people you set out to protect.

In fact, this situation is pretty much exactly what we see with other coronaviruses (the ones that cause the common cold) — most people eventually get infected at least once, because the virus comes around once a year.

Bottom Line

What does all of this mean? Well, endemic disease, while definitely a possibility, would probably not be a very good thing for COVID-19. Not only would it involve a huge number of younger people getting infected — while they might not be at as high a risk of death, they still face other potential issues from COVID-19 — it would probably ensure that most if not all older people got the disease as well. Many of them would die, as we know all too well. The thing about letting people get infected is that the logical endpoint of this strategy is, well, people getting infected.

It’s possible that a shielding strategy could be implemented long-term, of course. We could barricade anyone who has a >1% risk of death from COVID-19 (so, people over the age of 61) inside forever, against their presumably fervent objections. I suspect this would not be easy, and given the global experience of protecting elderly people may in fact not work anyway, but it is certainly something that we could try.

But a more realistic scenario is that, if we want to reduce the death toll from COVID-19, we are stuck with some public health activity for a time yet, at least until we have a safe and effective vaccine. Whether this looks like most states in Australia, where many restrictions are easing amid huge testing and contact tracing efforts, or Sweden where large gatherings and other events are still banned indefinitely and other restrictions are still in place, or even places like Vietnam, which has controlled the virus impressively well, there are many roadmaps to follow.

It is worth noting that none of these are “lockdowns” in any sense. In fact, you’d be hard-pressed to find a single epidemiologist anywhere in the world proposing a lockdown at this point. No one is more aware of the societal costs of health interventions than epidemiologists, and given the health consequences of protracted isolation we’d all like to avoid that if possible. I don’t think any long-term roadmaps include lockdowns moving forward, because they are costly interventions that we’d all rather avoid.

Regardless, whatever the future looks like, we can be fairly certain it won’t be herd immunity. The virus isn’t just going to magically disappear — even the people proposing strategies where most people get infected aren’t saying that COVID-19 is going to go away entirely. They are just arguing that the disease will become endemic, with regular outbreaks to keep us company well into the future.

Whether accepting COVID-19 as endemic is the best choice is, ultimately, not a scientific decision — it is a political one. Science can only give us the answers to factual questions, like “how many people over 60 will die if infected with COVID-19?”, it can’t answer the perhaps more challenging problems like how much government action we should use to control the disease. What we do know, however, is that allowing the coronavirus to infect large proportions of the population comes with a staggering human cost, and that fact has not changed.

Bottom line? There are many options for long-term control of COVID-19.

Herd immunity simply isn’t one of them.

If you enjoyed, follow me on Medium, Twitter or Facebook!