Herd immunity is a form of indirect protection from infectious disease. It occurs when a sufficient percentage of a population has become immune to an infection. The greater the proportion of immune individuals in a community, the smaller the probability that non-immune individuals will come into contact with an infectious individual. Once the herd immunity threshold has been reached, disease gradually disappears from a population.
About Herd immunity in brief
Herd immunity is a form of indirect protection from infectious disease. It occurs when a sufficient percentage of a population has become immune to an infection. The greater the proportion of immune individuals in a community, the smaller the probability that non-immune individuals will come into contact with an infectious individual. Some individuals cannot become immune because of medical conditions, such as an immunodeficiency or immunosuppression. Once the herd immunity threshold has been reached, disease gradually disappears from a population. If achieved worldwide, this elimination may result in the permanent reduction in the number of infections to zero, called eradication. Herd immunity was recognized as a naturally occurring phenomenon in the 1930s. Mass vaccination to induce herd immunity has since become common and has proved successful in preventing the spread of many infectious diseases. High levels of immunity in one age group can create herd immunity for other age groups. For sexually transmitted infections, high levels of immune in one sex induces herd immunity in both sexes. Vaccines against STIs that are targeted at one sex result in significant declines in vaccine uptake in both sex if the target sex is high. If vaccination uptake among the target sexes is low, then the other sex can be sufficiently protected to be immunized. High-risk behaviors make eliminating STIs difficult, even though most occur among individuals with moderate risk. In this case, in this case referred to as herd immunity, the majority of transmissions occur because individuals who engage in high-risk behaviours are able to escape from herd immunity.
For these reasons, in certain populations it may be necessary to immunize high- Risk persons or moderate-risk persons or high- risk persons of both sexes to escape herd immunity from certain strains of STIs. The exact herd immunityreshold varies depending on the disease. An example of a disease with a low threshold is influenza, with a HIT of 33–44%. A high threshold is the measles, with an HIT of 92–95%. Some individuals either cannot develop immunity after vaccination or for medical reasons cannot be vaccinated. A portion of those vaccinated may not develop long-term immunity. Vaccine contraindications may prevent certain individuals from being vaccinated. The prioritization of school-age children for seasonal flu immunization, which is more effective than vaccinating the elderly, has been shown to create a certain degree of protection for the elderly. Vaccinating adults against pertussis reduces pertussi incidence in infants too young to be vaccinated, who are at the greatest risk of complications. This is especially important for close family members, who account for most of the transmissions to young infants. Vaccinating children against pneumococcus and rotavirus reduces pneumococcal disease incidence among younger, unvaccinated siblings, who do not normally receive these vaccines. In the same manner, children receiving vaccines against pneumococcus has had the effect of reducing pneumococci-attributable hospitalizations for older children and adults, who don’t normally receive them.