Herd Immunity

Herd immunity describes a state in which an infectious disease transmissible through human contact is unlikely to spread because a large proportion of the population is immune to the disease. This immunity can be conferred through natural immunity, previous exposure to the disease, or vaccination. The entire population does not need to be immune to attain herd immunity. Rather, the population density of persons susceptible to infection must be low enough to minimize the likelihood of an infected individual coming in contact with a susceptible individual. Herd immunity can prevent sustained disease spread in populations, thereby protecting susceptible individuals from infection. It is important to emphasize that the concept of herd immunity is applicable only to infectious diseases that can be spread by human contact. For example, herd immunity is unlikely to protect unvaccinated persons from tetanus, due to the ubiquitous presence of Clostridium tetani in natural reservoirs of soil and animal droppings.

The percentage of the population that must be immune to produce herd immunity differs for each infectious disease. A disease with a high infectivity rate such as measles will require a higher proportion of immune persons to achieve herd immunity than a disease with lower infectivity such as tuberculosis. In addition, individualand population-level characteristics influencing disease spread such as susceptibility, demographics, social habits, and clustering affect herd immunity.

Herd immunity is an important consideration for mass vaccination practices. Even if a cheap, safe, and effective vaccine exists, resource, logistical, and societal constraints prevent the vaccination of 100% of the population. A reasonable target level of vaccination may be to achieve the threshold level of herd immunity H, which is calculated as

where R0 is the basic reproductive rate, the number of infections an infected individual can be expected to produce on entry into a susceptible population. Mass vaccination can be successful through principles of herd immunity, although disease outbreaks can still occur, although generally to a lesser extent than if herd immunity had not been achieved.