At Stockholm University, Tom Britton, the dean of mathematics and physics, thinks that a 20 percent threshold is unlikely, but not impossible. His lab has also been building epidemiological models based on data from around the globe. He believes that variation in susceptibility and exposure to the virus clearly seems to be reducing estimates for herd immunity. Britton and his colleagues recently published their model, demonstrating the effect, in Science.
“If there is a large variability of susceptibility among humans, then herd immunity could be as low as 20 percent,” Britton told me. But there’s reason to suspect that people do not have such dramatically disparate susceptibility to the coronavirus. High degrees of variability are more common in things such as sexually transmitted infections, where a person with 100 partners a year is far more susceptible than someone celibate. Respiratory viruses tend to be more equal-opportunity invaders. “I don’t think it will happen at 20 percent,” Britton said. “Between 35 and 45 percent—I think that would be a level where spreading drops drastically.”
Models like Britton’s and Gomes’s also assume that, after infection, people obtain immunity. This is a clear caveat that all the researchers make. COVID-19 is a new disease, so no one can be sure that infected people become immune reliably, or how long immunity lasts. But Britton noted that there are no clear instances of double infections so far, which suggests that this virus creates immunity for at least some meaningful length of time, as most viruses do.
Lipsitch also believes that heterogeneity is important to factor into any model. It was one reason he updated his prediction, not long after we spoke in February, of what the herd-immunity threshold would be. Instead of 40 to 70 percent, he lowered it to 20 to 60 percent. When we spoke last week, he said he still stands by that, but he is skeptical that the number lands close to the 20 percent end of the range. “I think it’s unlikely,” he said, but added, “This virus is proving there can be orders-of-magnitude differences in attack rates, depending on political and societal decisions, which I don’t know how to forecast.”
“I think we all agree that heterogeneity is important,” says Kate Langwig, a professor at Virginia Tech. She studied at Harvard under Lipsitch, and was also mentored by Gomes. Biological variations in susceptibility could come down to factors as simple as who has more nose hair, or who talks the loudest and most explosively, and Langwig shares the belief that these factors can create heterogeneity in susceptibility and transmission. Those effects can compound to dramatically change the math behind predictions for the future.
But she declines to endorse any particular numeric threshold for herd immunity. She’s not comfortable with the idea of a single number at all. What’s important to her, rather, is that people are not misled by the idea of herd immunity. In the context of vaccination, herd-immunity thresholds are relatively fixed and predictable. In the context of an ongoing pandemic, thinking of this threshold as some static concept can be dangerously misleading.