Covid-19 herd immunity, explained

Once the herd immunity threshold is reached, the virus just doesn’t go away.

Arthobbit via Getty Images

There are two ways to reach herd immunity for Covid-19: the slow way, and the catastrophic way.

How will the Covid-19 pandemic end? And when?

These have been the biggest questions since the pandemic began earlier this year. The answer likely depends on one routinely misinterpreted concept in public health: herd immunity.

“Herd immunity is the only way we’re going to move to a post-pandemic world,” says Bill Hanage, an epidemiology researcher at Harvard. “The problem is, how do you get to it?”

Typically, the term herd immunity is thought of in the context of vaccination campaigns against contagious viruses like measles. The concept helps public health officials think through the math of how many people in a population need to be vaccinated to prevent outbreaks.

With Covid-19, since we don’t yet have a vaccine, the discussion has centered on herd immunity through natural infection, which comes with a terrible cost. Confusing matters, too, is the persistent and erroneous wishful thinking by some who say herd immunity has already been reached, or will be reached sooner than scientists are saying.

For instance, at a recent Senate hearing, Sen. Rand Paul (R-KY) claimed that New York City has its outbreak under control thanks to herd immunity and the fact that around 22 percent of the city’s residents had been infected.

But Dr. Anthony Fauci of the National Institutes of Health, who was a panelist at the hearing, immediately spoke up to correct the senator: “If you believe 22 percent is herd immunity, I believe you’re alone in that.”

Hypothetically, yes, there are situations under which herd immunity to Covid-19 could be achieved. Manaus, Brazil, an Amazonian city of around 2 million people, experienced one of the most severe Covid-19 outbreaks in the world. At the peak in the spring and early summer, the city’s hospitals were completely full, the New York Times reported.

During this period, there were four times as many deaths as normal for that point in the year. But then, over the summer, the outbreak sharply died down. Researchers now estimate between 44 percent and 66 percent of the city’s population was infected with the virus, which means it’s possible herd immunity has been achieved there. (This research has yet to be peer-reviewed.)

But that’s much higher than 22 percent, and the cost of this herd immunity was immense: Between 1 in 500 and 1 in 800 residents died there, the researchers estimate.

Many more were hospitalized, and still more may suffer long-term consequences of the infection. Similarly, the oft-cited example of Sweden, which has pursued a laxer social distancing strategy than its European peers (partially with the goal of building up herd immunity in younger people, while protecting older residents and trying to keep hospitals from exceeding capacity), has paid a price, too: a much higher death rate than fellow Scandinavian countries.

We’re several months into this pandemic, and herd immunity is still widely misunderstood and being continually misused for partisan goals of discrediting science and scientists. The biggest misconception is that achieving herd immunity through natural infection is a reasonable pandemic response strategy. It’s not. Let’s explain.

Herd immunity, explained, simply

There’s a simple explanation of herd immunity.

After a certain proportion of a population has become immune to a virus, an outbreak will stop growing exponentially. There may continue to be new cases, but each new case will be less likely to start a big chain of infections.

In this simple view, the herd immunity threshold — that specific proportion of the population with some immunity — is derived from a value called the R0 (r-naught). This is the figure that quantifies the average transmissibility of a disease. If the R0 is 2, that means that, on average, each case will lead to fewer than two new cases.

So the herd immunity threshold for a disease of this contagiousness is 50 percent. When half the population becomes immune, then, the outbreak may start to decline because the virus will not be able to spread as easily. For Covid-19, the exact figure for the threshold depends on whom you ask. Based on the simple math, “the expectation for the natural herd immunity level for Covid would be 60 to 75 percent,” Shweta Bansal, a Georgetown University epidemiologist, says. Though the figure could be a bit lower, perhaps 40 percent, in some places.

Regardless of the exact figure, as a country, the US is nowhere near reaching this threshold. In New York City, which experienced the worst coronavirus outbreak in the US, around 20 percent of residents got infected and 23,000-plus people died. Overall, a new Lancet study — which drew its data from a sample of dialysis patients — suggests that fewer than 10 percent of people nationwide have been exposed to the virus. That means we have a long, sick, and deadly way to go if the US is going to reach herd immunity through natural infection.

So far, there have been more than 200,000 deaths in the United States, with relatively few infections. There’s so much more potential for death if the virus spreads to true herd immunity levels. “The cost of herd immunity [through natural infection] is extraordinarily high,” Hanage says.

The herd immunity threshold for Covid-19 could be lower, or higher, than 60 percent. It depends on the makeup of a community, and its social dynamics.

So that’s the simple math of herd immunity — it’s a fraction derived from the R0 of the virus. Easy, right? In reality, how herd immunity through natural infection plays out in the real world is much messier, and very hard to precisely predict.

For one, this simple mathematical view of herd immunity assumes that risk of catching the disease in a population is evenly distributed. But we know that isn’t the case with Covid-19.

Risk of catching the virus varies greatly and in a number of dimensions. Here, physician and virology expert Muge Cevik breaks down the dimensions of risk:

The risk of transmission is complex and multi-dimensional. It depends on many factors: contact pattern (duration, proximity, activity), individual factors, environment (i.e. outdoor, indoor) & socioeconomic factors (i.e. crowded housing, job insecurity). (2/n) pic.twitter.com/0mEiHhbnWa

— Muge Cevik (@mugecevik) September 21, 2020

As we’ve seen, some people are more at risk of infection and severe illness or death because of their job, the environments they live and work in, the makeup of their immune system, socioeconomic factors like poverty, or their behavior: Some people may be willfully disregarding social distancing and mask-wearing mandates.

Knowing that the population doesn’t evenly share risk means the threshold for herd immunity can change based on who gets infected. Let’s say all the people most at risk of both catching and spreading the virus all get infected first. Then “the immunity within that group will have a particularly outsize benefit,” Hanage says. “Because they are the core group driving infection.”

So the overall threshold for herd immunity will be lower. How much lower?

Some hypothetical estimates put it as low as 20 percent, but “I think that is a stretch,” Bansal says. “Is [the herd immunity threshold] lower than 60 percent? Sure, that’s entirely possible. But I again, I think I don’t want any place on the planet to get to anything even close to that, right, in terms of infection rates.”

Hanage underscores a gross inequality here: Herd immunity achieved through natural infection would come at an undue cost to some of the most vulnerable, marginalized groups in the country.

“Because of the fact that some groups are more at risk of becoming infected than others — and they are predominantly people from racial [and] ethnic minorities and predominantly poor people with less good housing — we are effectively forcing those people to have a higher risk of infection and bear the brunt of the pandemic,” Hanage says.

The herd immunity threshold can be lower than estimated. But hypothetically, the threshold could be higher as well. It’s also the case that the herd immunity threshold can change over time. Remember the simple math of how herd immunity calculated: The threshold is dependent on the contagiousness of the virus.

Well, the contagiousness of the virus isn’t a fixed biological constant. It’s the result of the biology of the virus interacting with human biology, with our environments, with our society. As seasons change, as our behavior changes, so can the transmissibility of the virus. The herd immunity threshold is not one fixed target.

Herd immunity doesn’t end the pandemic. It just slows it down.

Once you hit the herd immunity threshold, it doesn’t mean the pandemic is over. After the threshold is reached, “all it means is that on average, each infection causes less than one ongoing infection,” Hanage says. “That’s of limited use if you’ve already got a million people infected.” If each infection causes, on average, 0.8 new infections, the epidemic will slow. But 0.8 isn’t zero. If a million people are infected at the time herd immunity is hit, per Hanage’s example, those already infected people may infect 800,000 more.

There are a lot of other unknowns here, too. One is the type of immunity conferred by natural infection. “Immunity” is a catchall term that means many different things. It could mean true protection from getting infected with the virus a second time. Or it could mean reinfections are possible but less severe. You could, potentially, get infected a second time, never feel sick at all (thanks to a quick immune response), and still pass on the virus to another person. “If immunity just reduces disease … then [the] concept loses meaning,” Sarah Cobey, a computational biologist at the University of Chicago, writes in an email, noting, though, that this scenario is “unlikely.”

Overall, we don’t know if herd immunity through natural infection would look the same as herd immunity achieved through a vaccination campaign. “We don’t know yet if those two things will be different,” Christine Tedijanto, an epidemiology researcher at Harvard, says.

Even New York City could see another big wave

Right now, New York City appears to have its epidemic mostly under control, with fewer than 200 new cases a day, down from a springtime high of more than 5,000 cases per day. But the progress is precarious, with city health officials growing concerned about increasing clusters of cases in several of the city’s neighborhoods. Mayor Bill de Blasio said the city needs to take “urgent action” to prevent these clusters from growing.

It’s possible there are pockets of herd immunity in some New York communities, and, overall, it’s estimated around 20 percent of the city’s residents contracted the virus. Despite what Sen. Paul might think, New York has achieved some control through measures like social distancing and mask-wearing.

“As soon as they lift their foot off the brake, they will see that outbreak come back,” Bansal says. The reason New York has the epidemic under control is not because it’s achieved herd immunity; it’s because it’s gotten its act together.

But even if there’s some degree of protection in New York from the natural infections that have occurred there, that protection will only last while mitigation measures are in place.

Another way to think about it: Through control measures, New York City has successfully, and artificially, reduced the transmissibility of the virus. That temporarily lowers the bar for the herd immunity threshold. But the city can’t resume life as it went on before the pandemic struck. That would increase the transmissibility of the virus, and the epidemic would grow there until reaching a higher herd immunity threshold.

Also, in New York, it’s important to remember that the level of immunity could vary widely from one community to the next. “Even if one borough has reached a herd immunity threshold, the boroughs around it may not have,” Tedijanto says.

Why you can’t infect the young to protect the old

Let’s say herd immunity is achieved through millions of younger people getting sick. White House adviser Scott Atlas (who is a neuroradiologist, not an epidemiologist) has suggested this is a good thing to do. “When younger, healthier people get infected, that’s a good thing,” he said in a July interview with a San Diego local news station. “The goal is not to eliminate all cases. That’s not rational, not necessary if we just protect the people who are going to have serious complications.”

Let’s be clear, it’s not a “good thing” when young people get sick. For one, some of these young people may die, more may get severely ill, and a not-yet-understood proportion of them could suffer long-term consequences. Remember: The more people infected, the more chances for rare, horrible things to happen.

These younger people, now immune, could, in theory, protect older populations more at risk of dying from Covid-19. But in building up herd immunity in this way, we’ve also built up powder kegs of vulnerability among the older people, which can be set off in the future.

“I think it’s impossible to think that you can have infections only among younger people, and not let them spread to other groups with populations that might be more vulnerable,“ Tedijanto says. People just don’t separately themselves so neatly into risk groups like that.

“We can try and insulate” older people, Hanage says. “We can do a very good job of insulating them. But the fact is, the larger the amount of infection outside them, the higher the chance that something’s going to get into them.”

Overall, here’s the biggest problem with thinking about herd immunity through natural infection: It’s impossible to predict which route it is going to go. “We don’t understand and measure our world in very deep ways yet,” Bansal says. We can’t predict the movements and behaviors, the risk factors, of millions of people, and how they change over time. Allowing herd immunity to develop through natural infection means letting the virus rip a hard-to-predict course through the population.

Herd immunity isn’t a dirty word. When a vaccine comes, it will be essential for scientists to devise a strategy to most effectively inoculate the country and end the pandemic. The price of achieving herd immunity through a vaccine campaign is the price of the vaccine, and the price of our patience waiting for it.

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