Viruses require a host to replicate and survive. When the majority of a population develops antibodies against the virus and becomes resistant to an infectious disease, either through natural infection or by vaccination, the virus cannot spread effectively. Unable to find a suitable host, the virus cannot be transmitted because it has nowhere to go. Herd immunity protects the entire community by keeping the spread under control.
Herd immunity is achieved when the herd immunity threshold is reached.
- The herd immunity threshold is the proportion of the community that needs to be immune to the disease in order for transmission to decline.
- The herd immunity threshold is dependent on the degree of virus transmissibility which is represented by the value R0 (pronounced “R naught”).
- The R0 value indicates the number of individuals to whom the virus is transmitted by a single infected person.
- When R0 is greater than 1 infections will increase over time; when R0 is less than 1 infections will decrease.
- A higher R0 means a larger proportion of the population must be immune to stop transmission.
- For most infectious diseases, herd immunity can be achieved when 70-90% of the community is immune.
- By preventing viruses from propagating effectively, herd immunity keeps the disease spread under control.
- Because they cannot be vaccinated, herd immunity indirectly protects the most vulnerable members of the community (e.g., immunosuppressed individuals, people receiving chemotherapy, infants, critically ill patients, the elderly).
- While herd immunity does protect the community from outbreaks, it does not guarantee protection from the disease for individuals.
- Herd immunity is only effective against diseases that pass from person to person (e.g., measles). It does not protect the population against diseases that are contracted from the environment, such as tetanus.
- Due to various factors such as waning immunity, vaccine hesitancy, and disparities in vaccine distribution, total herd immunity is rarely achieved.