Herd immunity is a concept that refers to the indirect protection of individuals who are not immune to an infectious disease due to the presence of immune individuals in the population. This phenomenon reduces the risk of infection for susceptible individuals by limiting the spread of the disease. The term "herd immunity" has been used in various ways, but it generally implies that a sufficient proportion of the population must be immune to prevent the spread of an infection. The concept of herd immunity was first introduced over a century ago, but it gained more attention in recent decades due to the increasing use of vaccines and discussions about disease eradication. A key milestone was the recognition in the 1970s that if immunity is randomly distributed and individuals mix randomly, the incidence of an infection will decline when the proportion of immune individuals exceeds a threshold, calculated as (R0 - 1)/R0, where R0 is the basic reproduction number. Herd immunity has been observed in various contexts, such as the reduction of disease incidence in populations due to the indirect protection provided by vaccines. For example, conjugate vaccines against pneumococcal and Haemophilus infections have been shown to reduce disease incidence in older cohorts. Selective vaccination of high-risk groups can also reduce transmission and protect vulnerable populations. Theoretical developments have shown that herd immunity thresholds depend on factors such as the transmissibility of the infectious agent, the nature of the immunity induced by the vaccine, and the distribution of immunity in the population. Recent advances in modeling and data collection have improved our understanding of these complexities. Public health practice emphasizes the importance of maintaining high vaccination coverage to achieve herd immunity, even though it is not always possible to reach 100% coverage. Challenges include ensuring accurate monitoring of vaccination coverage, maintaining high levels of coverage as diseases decline in frequency, and addressing antivaccine sentiment. Herd immunity also raises ethical and legal considerations, as it involves the indirect protection of unvaccinated individuals, which can be seen as imposing a small risk on certain individuals for the benefit of others. Overall, herd immunity is a critical concept in public health, influencing vaccination strategies and disease control efforts.Herd immunity is a concept that refers to the indirect protection of individuals who are not immune to an infectious disease due to the presence of immune individuals in the population. This phenomenon reduces the risk of infection for susceptible individuals by limiting the spread of the disease. The term "herd immunity" has been used in various ways, but it generally implies that a sufficient proportion of the population must be immune to prevent the spread of an infection. The concept of herd immunity was first introduced over a century ago, but it gained more attention in recent decades due to the increasing use of vaccines and discussions about disease eradication. A key milestone was the recognition in the 1970s that if immunity is randomly distributed and individuals mix randomly, the incidence of an infection will decline when the proportion of immune individuals exceeds a threshold, calculated as (R0 - 1)/R0, where R0 is the basic reproduction number. Herd immunity has been observed in various contexts, such as the reduction of disease incidence in populations due to the indirect protection provided by vaccines. For example, conjugate vaccines against pneumococcal and Haemophilus infections have been shown to reduce disease incidence in older cohorts. Selective vaccination of high-risk groups can also reduce transmission and protect vulnerable populations. Theoretical developments have shown that herd immunity thresholds depend on factors such as the transmissibility of the infectious agent, the nature of the immunity induced by the vaccine, and the distribution of immunity in the population. Recent advances in modeling and data collection have improved our understanding of these complexities. Public health practice emphasizes the importance of maintaining high vaccination coverage to achieve herd immunity, even though it is not always possible to reach 100% coverage. Challenges include ensuring accurate monitoring of vaccination coverage, maintaining high levels of coverage as diseases decline in frequency, and addressing antivaccine sentiment. Herd immunity also raises ethical and legal considerations, as it involves the indirect protection of unvaccinated individuals, which can be seen as imposing a small risk on certain individuals for the benefit of others. Overall, herd immunity is a critical concept in public health, influencing vaccination strategies and disease control efforts.