How will the Covid-19 pandemic end? And when?
These have been the biggest questions since the pandemic began earlier this year. The answer probably hinges on a commonly misinterpreted public health concept: herd immunity.
“Herd immunity is the only way we will move into a post-pandemic world,” says Bill Hanage, an epidemiology researcher at Harvard. “The problem is, how do you get there?”
Typically, the term herd immunity is thought of in the context of vaccination campaigns against contagious viruses such as measles. The concept helps public health officials think about how many people in a population need to be vaccinated to prevent epidemics.
For example, in a recent Senate hearing, Senator Rand Paul (R-KY) said New York City has its outbreak under control thanks to herd immunity and the fact that about 22 percent of the city’s residents were been infected.
But Dr Anthony Fauci of the National Institutes of Health, who was a speaker at the hearing, immediately spoke to correct the senator: “If you believe 22 percent is herd immunity, I believe you are alone in that.”
Hypothetically, yes, there are situations where herd immunity to Covid-19 could be achieved. Manaus, Brazil, an Amazonian city of around 2 million people, has experienced one of the most serious Covid-19 outbreaks in the world. At the height of spring and early summer, the city’s hospitals were completely full, the New York Times reported.
During this time, there were four times more deaths than normal for that point of the year. But then, during the summer, the epidemic abruptly subsided. Researchers now estimate that between 44% and 66% of the city’s population have been infected with the virus, meaning it is possible that 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 others have been hospitalized and even more could suffer the long-term consequences of the infection. Likewise, the often cited example of Sweden, which has pursued a more lax social distancing strategy than its European peers (partly with the aim of strengthening herd immunity in young people, while protecting older residents and trying to prevent hospitals from exceeding capacity), it also paid a price: a much higher mortality rate than in other Scandinavian countries.
We have been in this pandemic several months ago and herd immunity is still widely misunderstood and is continuously misused for partisan purposes of discrediting science and scientists. The biggest misconception is that achieving herd immunity through natural infection is a reasonable pandemic response strategy. It is not. We explain.
Herd immunity, explained, simply
There is a simple explanation of herd immunity.
After a certain percentage of a population has become immune to a virus, an epidemic will stop growing exponentially. There may continue to be new cases, but each new case will be less likely to start a large 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 R0 (r-zero). This is the figure that quantifies the average transmissibility of a disease. If R0 is 2, it means that, on average, each case will lead to less 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 epidemic can begin to subside because the virus won’t be able to spread easily. For Covid-19, the exact figure for the threshold depends on who you ask. Based on simple calculations, “the expectation for the natural level of herd immunity for Covid would be 60 to 75 percent,” says Shweta Bansal, an epidemiologist at Georgetown University. Although the figure could be a little lower, perhaps 40 percent, in some places.
Regardless of the exact figure, as a country, the United States is nowhere near reaching this threshold. In New York City, which experienced the worst coronavirus outbreak in the United States, about 20 percent of residents were infected and more than 23,000 people died. Overall, a new Lancet study – which drew data from a sample of dialysis patients – suggests that fewer than 10 percent of people nationwide have been exposed to the virus. This means we have a long, sick and deadly road to go if the US is to achieve herd immunity through natural infection.
So far, there have been more than 200,000 deaths in the United States, with relatively few infections. There is much more to the potential for death if the virus spreads at true herd immunity levels. “The cost of herd immunity [through natural infection] he’s extraordinarily tall, “says Hanage.
The herd immunity threshold for Covid-19 could be below or above 60 percent. It depends on the structure of a community and its social dynamics.
This is the simple math of herd immunity: it is a fraction derived from the R0 of the virus. Easy, right? In reality, how herd immunity through natural infection unfolds in the real world is much more complicated and very difficult to predict accurately.
For one, this simple mathematical view of herd immunity assumes that the risk of contracting the disease in a population is evenly distributed. But we know that is not the case with Covid-19.
The risk of contracting the virus varies greatly and over a range of sizes. Here, physician and virology expert Muge Cevik breaks down the dimensions of the risk:
The risk of transmission is complex and multidimensional. It depends on many factors: contact pattern (duration, proximity, activity), individual factors, environment (eg external, internal) and socio-economic factors (eg crowded housing, job insecurity). (2 / n) pic.twitter.com/0mEiHhbnWa
– Muge Cevik (@mugecevik) 21 September 2020
As we have seen, some people are more at risk of infection and serious illness or death due to their jobs, the environments in which they live and work, the composition of their immune systems, socioeconomic factors such as poverty or their behavior: some people may intentionally ignore social distances and the mandates to wear the mask.
Knowing that the population does not share the risk equally means that the threshold for herd immunity can change based on who gets infected. Suppose all the people most at risk of both contracting and spreading the virus are infected first. So “immunity within that group will have a particularly huge advantage,” says Hanage. “Because they are the main group driving the infection.”
So the overall threshold for herd immunity will be lower. How much lower?
Some hypothetical estimates put it as low as 20%, but “I think that’s a stretch,” says Bansal. “IS [the herd immunity threshold] less than 60 percent? Of course, it is quite possible. But again, I think I don’t want any place on the planet to come close to that, right, in terms of infection rates. ”
Hanage points out here a grave inequality: herd immunity obtained through natural infection would have an undue cost to some of the most vulnerable and marginalized groups in the country.
“Due to the fact that some groups are more at risk of getting the infection than others – and they are mostly purebred people [and] ethnic minorities and predominantly poor people with poorer housing: we are effectively forcing those people to have a higher risk of infection and bear the brunt of the pandemic, “Hanage says.
Herd immunity threshold may be lower than estimated. But hypothetically, the threshold could be higher also. It is also true that the herd’s immunity threshold can change over time. Remember the simple math of how herd immunity is calculated: the threshold depends on the contagiousness of the virus.
Well, the contagiousness of the virus is not a fixed biological constant. It is the result of the biology of the virus interacting with human biology, with our environments, with our society. As the seasons change, as our behavior changes, the transmissibility of the virus can change. The herd immunity threshold is not a fixed target.
Herd immunity does not end the pandemic. It just slows it down.
Once the herd immunity threshold is reached, that doesn’t mean the pandemic is over. Once the threshold is reached, “all it means is that on average each infection causes less than an ongoing infection,” Hanage says. “It’s of limited use if you’ve already infected a million people.” If each infection causes, on average, 0.8 new infections, the outbreak will slow down. But 0.8 is not zero. If one million people are infected by the time the herd’s immunity is affected, according to Hanage’s example, those already infected people could infect another 800,000.
There are many other unknowns here too. One is the type of immunity conferred by natural infection. “Immunity” is an umbrella term that means many different things. It could mean real protection from getting infected with the virus a second time. Or it could mean that reinfections are possible but less severe. You could potentially get infected a second time, never feel sick (thanks to a quick immune response), and still pass the virus to another person. “If immunity only reduces the disease … then [the] the concept loses meaning, “writes Sarah Cobey, a computational biologist at the University of Chicago, noting, however, that this scenario is” unlikely “.
Overall, we don’t know if herd immunity through natural infection would be the same as herd immunity achieved through a vaccination campaign. “We still don’t know if those two things will be different,” says Christine Tedijanto, an epidemiology researcher at Harvard.
New York City could also see another big wave
Right now, New York City appears to have its epidemic mostly under control, with fewer than 200 new cases per day, down from a spring high of more than 5,000 cases per day. But progress is precarious, with city health officials growing concerned about the rise in case clusters in several city neighborhoods. Mayor Bill de Blasio said the city must take “urgent action” to prevent the growth of these clusters.
It is possible that there are pockets of herd immunity in some New York communities, and overall, an estimated 20% of the city’s residents are estimated to have contracted the virus. Despite what Senator Paul might think, New York has gained some control through measures such as social distancing and wearing masks.
“As soon as they take their foot off the brake, they will see the epidemic return,” says Bansal. The reason New York is keeping the epidemic in check is not because it has gained herd immunity; it’s because he put his actions together.
But even if there is some degree of protection in New York from the natural infections that have occurred there, that protection will only last as long as 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 lower the bar for the herd immunity threshold. But the city cannot resume life as it was before the pandemic broke out. This would increase the transmissibility of the virus and the epidemic would grow there until a higher herd immunity threshold was reached.
Also, in New York, it is important to remember that the level of immunity can vary greatly from one community to another. “Even if a district has reached a herd immunity threshold, surrounding municipalities may not have it,” says Tedijanto.
Because you can’t infect the young to protect the old
Suppose herd immunity is achieved through millions of young people who fall ill. White House Advisor 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 an interview in July with a local San Diego news station. “The goal is not to eliminate all cases. It is not rational, it is not necessary if we only protect people who will have serious complications. ”
Let me be clear, it is not a “good thing” when young people get sick. For one thing, some of these young people could die, others could become seriously ill, and some not yet understood could suffer long-term consequences. Remember: the more infected people are, the greater the chances of rare and horrible things happening.
These younger, now immune, people could, in theory, protect the older populations most at risk of dying from Covid-19. But in building herd immunity in this way, we have also built a powder keg of vulnerability among older people, which can be triggered in the future.
“I think it’s impossible to think that you can only have infections among young people and not let them spread to other groups with populations that might be more vulnerable,” says Tedijanto. People don’t separate so neatly into risk groups like that.
“We can try to isolate” older people, Hanage says. “We can do a great job isolating them. But the fact is, the greater the amount of infection outside of them, the greater the chance of something getting into them. “
Overall, here’s the biggest problem with thinking about herd immunity through natural infection: it’s impossible to predict which path it will take. “We don’t understand and measure our world very deeply yet,” says Bansal. We cannot 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 path through the population.
Herd immunity is not a bad word. When a vaccine arrives, it will be essential for scientists to devise a strategy to most effectively inoculate the country and end the pandemic. The price of obtaining herd immunity through a vaccine campaign is the price of the vaccine and the price of our waiting patience.
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