Herd immunity, also known as population immunity, is a form of indirect protection from infectious diseases that occurs when a sufficient percentage of a population has become immune to an infection, whether through previous infections or vaccination. This level of immunity in the community significantly reduces the likelihood of infection for individuals who are not immune, because the spread of contagious diseases is kept in check. The concept relies heavily on the principle that the pathogens responsible for disease transmission need susceptible hosts to propagate. When a high percentage of the community is immune, the pathogen has fewer opportunities to spread, leading to a decrease in the overall incidence of the disease.
The threshold for achieving herd immunity varies from one disease to another and is determined by the basic reproduction number, R0, which represents the average number of people to whom a single infected person will transmit the virus in a population that is entirely susceptible. For diseases with higher R0 values, such as measles, which can have an R0 of 12 to 18, a very high percentage of the population (typically 95% or more) needs to be immune to effectively break the chain of transmission. On the other hand, diseases with lower R0 values require a smaller proportion of the population to be immune to achieve herd immunity. This threshold ensures that each infected person transmits the disease to less than one other individual, thereby gradually leading to the disease's eradication within the community.
Achieving herd immunity can be done through natural infection or vaccination, but the latter is preferred for its control and predictability, and because it avoids the excessive morbidity and mortality associated with natural infections. Vaccination not only protects those who receive the vaccine but also those who cannot be vaccinated, such as newborns and those with certain health conditions. This protective mechanism is particularly important for preventing outbreaks of vaccine-preventable diseases in communities. It underscores the importance of high vaccination rates to maintain public health and prevent the resurgence of infectious diseases.
Despite its benefits, herd immunity faces several challenges and misconceptions. One significant challenge is the antivaccination movement, which can lead to lower vaccination rates and subsequently reduce community immunity. Moreover, the emergence of new pathogen strains, such as those seen with the influenza virus, can complicate the achievement of herd immunity because the immunity developed from previous infections or vaccinations might not be effective against new variants. Additionally, global travel means that diseases have more opportunities to cross borders, underscoring the need for widespread immunization efforts and international cooperation in public health. Herd immunity is not just a national concern but a global public health goal that requires collaborative efforts to manage and prevent infectious diseases worldwide.