 Infectious diseases are caused by pathogens like viruses or bacteria that can jump from one person to the next. So how can we stop the cycle of transmission? Simple answer is immunity. Immunity is our body's ability to resist a pathogen via the immune system. Today we'll talk about herd immunity. Immunity zoomed out to the level of the population. To understand herd immunity, we first need to understand how we become immune to pathogens and how herd immunity works under ideal circumstances. Our bodies can gain immunity to pathogens in one of two ways, by getting and recovering from an infection, also known as natural immunity, or by receiving a vaccination. In both cases, our bodies generate a type of immunity called acquired immunity, which is like a protective shield for our bodies against outside invaders, similar to a moat around a castle. Acquired immunity can give us long lasting immune protection through antibodies, proteins that help our immune systems recognize and destroy pathogens. For infectious diseases, this not only protects you from getting sick when you're exposed to the pathogen for a second time, but can also prevent you from spreading the disease to other people. This is a great way to prevent the spread of disease because the more people in a population who become immune, the less that pathogen is able to jump from person to person, essentially breaking the cycle of transmission. When enough people in a population become immune, the rest of the population is protected by proxy, establishing herd immunity. This is particularly important for people whose immune systems are too fragile to develop natural immunity or get a vaccination, like the elderly, the very young, or those who are immunocompromised. Let's take a look at this idea visually, considering a viral infection. When a new virus enters a population with no immunity, it can spread unchecked because the people in that population don't have the protective antibodies. As more people gain immunity, the number of infected individuals diminishes. This decreases the likelihood that an infected person will transmit the disease to someone susceptible. What happens as this continues? At a certain point, the virus runs out of places to go, and the spread of the virus is under control. Even those who haven't been infected or vaccinated are protected. We have achieved herd immunity. Achieving herd immunity requires the number of immune individuals to reach a certain threshold. So what is that threshold? The herd immunity threshold is calculated based on a number called R0, which refers to the average number of people an infected person spreads the disease to in a completely susceptible population. Using the estimated R0 for a pathogen, we can estimate the number of people who need to become immune for herd immunity to take effect. So the higher the R0, the more people who need to develop immunity to block sustained transmission. Let's take a look at this visually. For a pathogen that spreads very easily or is highly communicable, say one with an R0 of 5, many more individuals need to become immune to reach the herd immunity threshold. For a pathogen that spreads relatively more slowly, one with an R0 of 2 for example, less people need to develop immunity before herd immunity becomes protective. For the COVID-19 pandemic, R0 is approximately 3. And our mathematical model to calculate the herd immunity threshold tells us that theoretically two-thirds of the population would have to become immune in order to slow down and eventually stop the spread of SARS-CoV-2, the virus that causes COVID-19. In a population of 300 million, for example, 200 million people would need to be immune to reach the herd immunity threshold. So how infectious a pathogen is matters. Another important variable is time. Although immunity to some pathogens is long-lasting, and in some cases lifelong, immunity to other pathogens can wane over time, either because the pathogen mutates to avoid the antibodies or because the cells that make protective antibodies are lost. Short-lived immunity means that in some cases herd immunity cannot be achieved because there is always a large proportion of the population that is susceptible. What does this mean for COVID-19? We don't know yet. As we are still trying to understand how long natural immunity to SARS-CoV-2 lasts and whether we can elicit long-term immunity with a SARS-CoV-2 vaccine, long-term immunity is key to determining if herd immunity will be protective for COVID-19. Let's recap. Heard immunity is the indirect protection from infection conferred to susceptible individuals when a sufficiently large proportion of immune individuals exist in a population. Achieving herd immunity is a complex and dynamic process. Dependent on multiple factors, including the nature of the infectious pathogen and the type of immunity it generates. Heard immunity cannot quell a disease outbreak, but it is a critical way to prevent the spread and damage of an infectious agent across the global community. For some pathogens, the herd immunity threshold can be gradually established via natural infection. But for most, effective vaccinations are critical to reach the numbers necessary to achieve herd immunity. You can learn more about herd immunity in the primer by Luis Berrero and Hailey Randolph in the journal Immunity. Thank you for joining us today. We are Eureka Science and this is a collaboration with Cell Press.