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.””

My patient caught Covid-19 twice. So long to herd immunity hopes?

““Wait. I can catch Covid twice?” my 50-year-old patient asked in disbelief. It was the beginning of July, and he had just tested positive for SARS-CoV-2, the virus that causes Covid-19, for a second time — three months after a previous infection.

While there’s still much we don’t understand about immunity to this new illness, a small but growing number of cases like his suggest the answer is yes.

Covid-19 may also be much worse the second time around. During his first infection, my patient experienced a mild cough and sore throat. His second infection, in contrast, was marked by a high fever, shortness of breath, and hypoxia, resulting in multiple trips to the hospital.

Recent reports and conversations with physician colleagues suggest my patient is not alone. Two patients in New Jersey, for instance, appear to have contracted Covid-19 a second time almost two months after fully recovering from their first infection. Daniel Griffin, a physician and researcher at Columbia University in New York, recently described a case of presumed reinfection on the This Week in Virology podcast.

It is possible, but unlikely, that my patient had a single infection that lasted three months. Some Covid-19 patients (now dubbed “long haulers”) do appear to suffer persistent infections and symptoms.

My patient, however, cleared his infection — he had two negative PCR tests after his first infection — and felt healthy for nearly six weeks.”

“repeat infections in a short period are a feature of many viruses, including other coronaviruses. So if some Covid-19 patients are getting reinfected after a second exposure, it would not be particularly unusual.”

“Herd immunity depends on the theory that our immune systems, once exposed to a pathogen, will collectively protect us as a community from reinfection and further spread.”

“Experts generally consider natural herd immunity a worst-case-scenario backup plan. It requires mass infection (and, in the case of Covid-19, massive loss of life because of the disease’s fatality rate) before protection takes hold.”

What’s the Herd Immunity Threshold for the COVID-19 Coronavirus?

“Herd immunity is the resistance to the spread of a contagious disease that results if a sufficiently high proportion of a population is immune to the illness. Some people are still susceptible, but they are surrounded by immune individuals, who serve as a barrier preventing the microbes from reaching them. You can achieve this through either mass infection or mass vaccination.”

“Most the evidence so far suggests that people who recover from a COVID-19 coronavirus infection do, at least for a time, develop immunity to the microbe. If that’s true, what is the disease-induced herd immunity threshold for the COVID-19 coronavirus? Various epidemiologists offer different answers, depending upon their estimates for the disease’s R0 and other variables, but most have converged on a threshold at around 60 to 70 percent.

More recently, some researchers have suggested that this threshold may be too high. In a new preprint, three mathematicians from Sweden and the United Kingdom, using an R0 of 2.5, calculate a reduction in the herd immunity threshold from 60 percent to 43 percent by incorporating some assumptions with respect to populations’ social activity levels and age structures.

A couple of new reports speculatively lower the possible herd immunity threshold for the coronavirus to just 10 to 20 percent of the population. This conjecture depends chiefly on assumptions about just how susceptible and connected members of the herd are.”

“There are no solid estimates for the percentage of the U.S. population that has already been infected by the coronavirus, but Youyang Gu and his team at COVID19-Projections estimate that right now the number is between 2.2 to 4.7 percent. That would mean that somewhere between 7.3 and 15.5 million Americans have been infected. A similar result emerges from a very rough calculation that multiplies the number of confirmed cases at 1.4 million by a 10-fold factor of undiagnosed cases and infections. (The 10-fold factor is derived from data recently reported by Indiana University researchers.)
The upshot: The U.S. as a whole is not yet close to achieving even the speculatively low estimate of the herd immunity threshold.”