If we cannot stop COVID-19, can we at least look forward to the point in time when we are all survivors of the disease, and presumably immune? It’s a tempting idea, but fatally flawed. Herd immunity can protect individuals, but it won’t keep a population safe from a disease that has no vaccine.
Does that sound counterintuitive? Let’s take a little tour of what herd immunity actually is, how we’d achieve it, and what the result might be.
What is herd immunity?
As a contagious, deadly disease rips through a population, it leaves two things in its wake: dead bodies and survivors. For many diseases, including (we think) COVID-19, survivors are immune and cannot get the disease again.
For an individual person, immunity is protective. If I am immune to COVID-19, I can go out and not worry about being sick. If everyone else in the world is immune to COVID-19, I am still safe even if I am not immune myself.
The idea of being protected by the immunity of those around you is herd immunity. It’s a good thing when we’re talking about vaccination. Let’s say 95% of people in your town are immunized against measles. If a traveller with measles comes to visit, the disease won’t have many opportunities to spread to another person. Vaccines protect most people in the town, and the few people who weren’t vaccinated are protected by the simple fact that the disease doesn’t have a way to get to them.
Herd immunity doesn’t guarantee that everyone will be safe, but it does mean that, on average, outbreaks will fizzle out quickly.
The percentage of people who need to be immune to achieve herd immunity depends on the disease. Measles is very contagious: Each infected person could spread it to 12 to 18 other people in a population where nobody is immune. COVID-19 doesn’t spread as easily. The exact number, or R0, is still being determined, although it’s estimated to be in the range of 2 to 3. That means we might be able to achieve herd immunity with only, say, 60% of the population being immune.
Sounds great, let’s go!
Not so fast. “All this talk of ‘Oh once we get to herd immunity’ fails to recognise that the way in which we get herd immunity is that everybody gets sick,” says Ellie Murray, an assistant professor of epidemiology at Boston University School of Public Health.
It might be possible to be infected with the coronavirus without getting very sick, and Murray is quick to note that, but she also pointed out that many people who get infected get very sick. Even those who survive may spend weeks severely ill, may need hospital care, and may suffer organ damage or other long-term consequences. We don’t even know what all those consequences might be, since the first people in history to suffer the disease are only a few months into their recovery.
And people would die. In the US, 2.7% of people who get sick with COVID-19 die from it. That number is still tentative, and it varies by country and by age group, but when you apply it to the entire United States, you get millions of deaths.
Herd immunity protects us against measles because, thanks to the vaccine, we don’t need to get measles to become immune. For a disease like COVID-19 that has no vaccine, the only way to become immune is to become infected.
In short, “let’s get everybody sick” is not a solution to the problem of “how do we avoid getting everybody sick?”
Didn’t this work in Sweden, though?
Sweden famously chose a herd immunity approach, which some American politicians point to as a strategy that worked. But Sweden suffered far more deaths than neighbouring countries, nearly as many per capita as the United States. and they also had a number of advantages that the US does not have. While some call the strategy a success, others can fairly say it looks like a “disaster.”
The editor of the BMJ, a medical journal, writes that Sweden may have avoided a second wave, not because their herd immunity strategy worked, but possibly because “Swedes have followed the rules more dutifully than in other countries — perhaps because they trust the public health officials, understand what they have to do and why, and have avoided the stop-go confusion that besets Britain and other countries.”
Sweden’s approach was not to ignore the virus completely. They banned all gatherings of more than 50 people throughout the country. Workplaces and public transit were required to provide extra space for distancing. Sporting events were held without spectators. People with a COVID diagnosis were strongly discouraged from travelling.
Sweden also has a policy that people who stay home from work because they are sick are paid 80% of their salary. This makes for a huge difference in the likelihood that a sick worker will stay home. In the US, not only do many of us go to work sick for fear of missing income, there are often extra financial incentives to show up to work whether you are sick or not.
The herd immunity plan currently being discussed by the Trump administration (renamed “focused protection”) does not appear to involve paid sick leave or strict social distancing regulation.
But if we did all get the disease, at least we’d be immune, right?
Well, maybe. For a little while. Remember, we don’t know how long immunity to COVID-19 would last. Based on what scientists know of other coronaviruses, COVID-19 immunity might only last a year or so.
Even if it turned out that survivors are immune for life, there’s another problem. No community would be made of a majority of survivors for very long. Travellers come to visit; babies are born. Before measles vaccination, measles epidemics would occur every few years. That’s because it only took a few thousand babies to add enough susceptible people to the population to allow outbreaks to happen again. The numbers would be different with COVID-19, but the principle remains.
What if we only infect young, healthy people?
The first problem in that question is the word “we.” Who is doing the infecting? Who is signing people up for this experiment that certainly will have a death rate? That’s not ethical in the least.
But sure, let’s assume we’re ok with a bunch of people dying unnecessarily. (I, for the record, am not.) Is there a way to infect just the people who are most likely to survive a case of COVID-19? That’s the idea behind proposals to reopen schools, or to allow young people to go back to work while keeping older folks sequestered.
The problem with that, says Murray, is that herd immunity does not apply to a population as a whole. It only has meaning within networks of contacts. If your grandpa mostly interacts with people in his assisted living home, and none of them have immunity to the virus, then it just takes one infected visitor — you? — to spark an outbreak that takes off through the entire building.
There’s a further problem. Just because we want to infect 60% (or whatever is the herd immunity threshold) of the population, does not mean we can somehow control the outbreak and stop it when it reaches 60%. Epidemics have momentum, and an epidemic that has already infected more than half the population is just going to keep going. After all, if we’re willing and able to stop the outbreak, why wouldn’t we have stopped it at zero per cent, before it kills anybody?
And there’s that “we” again. It’s deeply unethical to send whole swaths of the population to their deaths, even if statistically we don’t know who they are. Coercing or asking for volunteers doesn’t meaningfully dodge this problem, and the idea of creating a social structure based on immunity has a terrible, unjust history.
If we do establish herd immunity, it will come at a great cost of human lives. It will likely not be effective for long. And it will not be a way of winning this ordeal against the virus; it will be a confirmation that we have lost.
This post was originally published in April 2020 and was updated on October 14, 2020 with new numbers on the case-fatality rate and a section on what we have learned from Sweden’s approach.