While herd immunity has successfully protected populations from other infectious diseases, such as measles and smallpox, all efforts to stop the SARS-CoV-2 virus — the virus behind COVID-19 — from spreading have fallen short. According to Giurgea, immune responses against this virus, whether achieved through vaccination or a previous infection, subside with time. Additionally, the ability of the virus to mutate allows it to evade these immune responses and continue to spread in the population.