According to the standard models, about 60 percent of the US population would need to be vaccinated against Covid-19 or recover from it to slow and ultimately stop the spread of the disease. But many experts I talked to suspect that the herd immunity threshold for naturally acquired immunity is lower than that.
"My guess would be it's potentially between 40 and 50 percent," Pitzer said.
Lipsitch agrees: "If I had to make a guess, I'd probably put it at about 50 percent."
These are mostly just educated estimates, because it's so hard to quantify what makes one person more susceptible than another. Many of the characteristics you might think to assign someone-like how much social distancing they're doing-can change from week to week.
"The whole heterogeneity problem only works if the sources of heterogeneity are long-term properties of a person. If it's being in a bar, that's not in itself sustained enough to be a source of heterogeneity," Lipsitch said.
Heterogeneity may be hard to estimate, but it's also an important factor in determining what the herd immunity threshold really is. Langwig believes that the epidemiological community hasn't done enough to try and get it right.
"We've kind of been a little sloppy in thinking about herd immunity," she said. "This variability really matters, and we need to be careful to be more accurate about what the herd immunity threshold is."
Some recent papers have tried. In June the journal Science published a study that incorporated a modest degree of heterogeneity and estimated the herd immunity threshold for Covid-19 at 43 percent across broad populations. But one of the study's coauthors, Tom Britton of Stockholm University, thinks there are additional sources of heterogeneity their model doesn't account for.
"If anything, I'd think the difference is bigger, so that in fact the herd immunity level is probably a bit smaller than 43 percent," Britton said.
Another new study takes a different approach to estimating differences in susceptibility to Covid-19 and puts the herd immunity threshold even lower. The paper's 10 authors, who include Gomes and Langwig, estimate that the threshold for naturally acquired herd immunity to Covid-19 could be as low as 20 percent of the population. If that's the case, the hardest-hit places in the world may be nearing it.
"We're getting to the conclusion that the most affected regions, like Madrid, may be close to reaching herd immunity," said Gomes. An early version of the paper was posted in May, and the authors are currently working on an updated version, which they anticipate posting soon. This version will include herd immunity estimates for Spain, Portugal, Belgium and England.
Many experts, however, consider these new studies-not all of which have been peer-reviewed yet-to be unreliable.
In a Twitter thread in May, Dean emphasized that there's too much uncertainty around basic aspects of the disease-from the different values of R 0 in different settings to the effects of relaxing social distancing-to place much confidence in exact herd immunity thresholds. The threshold could be one number as long as a lot of people are wearing masks and avoiding large gatherings, and another much higher number if and when people let their guard down.
Other epidemiologists are also skeptical of the low numbers. Jeffrey Shaman of Columbia University said that 20 percent herd immunity "is not consistent with other respiratory viruses. It's not consistent with the flu. So why would it behave differently for one respiratory virus versus another? I don't get that."
Miller added, "I think the herd immunity threshold [for naturally acquired immunity] is less than 60 percent, but I don't see clear evidence that any [place] is close to it."
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