The concept of herd immunity requires familiarity with some terms:
Infectious individual: A person harbouring an infectious disease agent (bacteria/ virus, etc.) who is capable of transmitting the infection to another person/ animal. When infected persons manifest signs/ symptoms of disease, they are referred to as ‘clinical cases’. However, a person may be infectious without manifesting any features of disease. Such individuals are referred to as ‘sub-clinical cases’, and play an important role in maintaining disease transmission.
Primary case: The first case of the disease in question
Secondary case: A person who contracts infection from a primary case
Serial interval: This is the average time interval between successive cases in a chain of transmission.
Susceptible population: This refers to those members of a population that are vulnerable to acquire infection due to a relative or absolute lack of immunity against the disease in question. The lack of immunity may be due to
- Absence of prior natural infection. Some infectious agents (viruses, bacteria) have multiple disease causing strains. In some, infection with any strain protects against infection with other strains. This is termed ‘cross-immunity’. However, in others, infection with one strain results in immunity against that strain only, leaving the person vulnerable to infection from any of the other strains.
- Waning of prior immunity. This could be because
- maternal immunity (which protects infants for the first six months or so) waned
- previous natural infection did not result in life-long immunity, and immunity waned over time
- immunity from previous vaccination waned over time, and the person was not revaccinated
- Immunocompromised states (diabetes mellitus, HIV, steroid therapy, cancer treatment, etc.)
- Unvaccinated state. This is applicable to conditions for which there is an effective vaccine. Even so, no vaccine is 100% effective. Therefore, a proportion of vaccine recipients will be vulnerable to natural infection despite their prior vaccination.
Effective contact: The type of contact necessary for transmission of the infection in question. Naturally, this would vary by disease- some infections require direct physical contact with an infectious person, while others (like respiratory infections) don’t.
Contact rate: The proportion of all possible contacts between susceptible and infectious individuals that lead to new infections. If 10 out of 100 contacts between susceptible and infectious individuals lead to new infections, the contact rate would be 0.1.
Basic case reproduction rate (R0): The average number of secondary cases that occur due to the introduction of a single primary case into a totally susceptible population. The number of secondary cases occurring from every primary case will not always be equal to the R0 (if the R0 is 2, it means that on average two secondary cases will occur from one primary case; not that every primary case will cause two secondary cases- if one primary case causes one secondary case, and another primary case causes three secondary cases, the average number of secondary cases per primary case (R0) will be two).