What people get wrong about herd immunity, explained by epidemiologists

““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.”

“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.” 

“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.”

“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 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.” 

“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.”

““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.””


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