What is herd immunity?
The basic idea is that a group (the “herd”) can avoid exposure to a disease by ensuring that enough people are immune so that no sustained chains of transmission can be established. This protects an entire population, especially those who are too young or too sick to be vaccinated.
Herd immunity is what happens when enough people are immune to a disease (in this case, COVID-19) for the chains of transmission to be broken. As more people become immune, infected people are less able to pass on the disease, and the spread of the disease slows down. This provides an indirect form of protection for those who are not immune.
Usually, individuals become immune through vaccination. But there isn’t yet a vaccine for coronavirus, so herd immunity could only currently be achieved if enough people contract the disease and recover, developing an immunity to it in the process.