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. At that point, whether it’s brought on by a vaccine or by people catching the disease, the population has developed “herd immunity.” “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. Kevin Hartnett writes in Quanta Magazine that 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.