This could be true in places like nursing homes, where the average person might be more susceptible to COVID-19 than the average person in the broader population.īut on a larger scale, heterogeneity typically lowers the herd immunity threshold. In some cases it will make the threshold higher. However, in an ongoing pandemic with no guarantee that a vaccine will be available anytime soon, the heterogeneity of susceptibility has real implications for the disease’s herd immunity threshold. Instead, health officials take a maximalist approach and, in essence, vaccinate everyone. “Vaccines are generally not distributed in a population with respect to how many contacts people have or how susceptible they are, because we don’t know that,” said Virginia Pitzer of the Yale School of Public Health. “This affects how able people are to fight a virus.”Įpidemiologists refer to these variations as the “heterogeneity of susceptibility,” meaning the differences that cause some people to be more or less likely to get infected.īut this is too much nuance for vaccination campaigns.
“We are born different, and then these differences accumulate as we live different experiences,” said Gabriela Gomes of the University of Strathclyde in Scotland. Biological differences also play a role in how likely people are to get infected. Differences in social behaviors lead some people to have more exposure to a disease than others. But in a naturally spreading infection, that’s not necessarily the case. Usually, researchers only think about herd immunity in the context of vaccine campaigns, many of which assume that everyone is equally likely to contract and spread a disease. average.Īnd just as R 0 turns out to be a variable, and not a static number, the way people acquire their immunity also varies, with important implications for calculating that herd immunity threshold. He cited data indicating it could be more than twice as high in some urban settings as the overall U.S. “I think the range of R 0 consistent with data for COVID-19 is larger than most people give credit to,” said Marc Lipsitch of Harvard University, who has been advising health officials in Massachusetts and abroad. This means that the herd immunity threshold will also be higher than 60% in some places and lower in others. So while an R 0 of 2.5 for COVID-19 may be a reasonable number for the whole world, it will almost certainly vary considerably on a more local level, averaging much higher in some places and lower in others. The average infected person in an apartment building may infect many more people than the average infected person in a rural setting. The herd immunity threshold depends on how many people each infected person actually infects - a number that can vary by location. “It doesn’t mean you won’t be able to start a fire at all, but that outbreak is going to die,” said Kate Langwig of Virginia Polytechnic Institute and State University. That means the virus will spread at an accelerating rate until, on average across different places, 60% of the population becomes immune. In that case, the herd immunity threshold for COVID-19 is 0.6, or 60%. Let’s say the R 0 for COVID-19 is 2.5, meaning each infected person infects, on average, two and a half other people (a common estimate). Once you have that, you can plug it into a simple formula for calculating the herd immunity threshold: 1 − 1/R 0. This value is called R 0 (pronounced “R naught”).
The only thing you need to know is how many people, on average, are infected by each infected person. While determining that threshold for COVID-19 is critical, a lot of nuance is involved in calculating exactly how much of the population needs to be immune for herd immunity to take effect and protect the people who aren’t immune.Īt first it seems simple enough. “Once the level of immunity passes a certain threshold, then the epidemic will start to die out because there aren’t enough new people to infect,” said Natalie Dean of the University of Florida. At that point, whether it’s brought on by a vaccine or by people catching the disease, the population has developed “herd immunity.” While much about the COVID-19 pandemic remains uncertain, we know how it will likely end: when the spread of the virus starts to slow (and eventually ceases altogether) because enough people have developed immunity to it.