Fjöldónæmi

Fjöldónæmi, also known as herd immunity or community immunity, is a concept in epidemiology where a sufficiently large proportion of a population is immune to a specific infectious disease, thereby protecting individuals who are not immune. This protection arises because the spread of the disease is significantly hampered or stopped altogether. When most individuals are immune, the chances of an infected person encountering a susceptible person are drastically reduced, breaking the chain of transmission.

The level of immunity required to achieve fjöldónæmi, known as the herd immunity threshold, varies depending on the contagiousness of the disease. Highly contagious diseases like measles require a very high percentage of immune individuals, often above 90%. Less contagious diseases may have a lower threshold. Immunity can be acquired through natural infection and recovery or through vaccination. Vaccination is considered a safer and more predictable way to achieve fjöldónæmi without the risks associated with natural infection.

Fjöldónæmi is crucial for protecting vulnerable populations who cannot be vaccinated or for whom vaccines are less effective. This includes infants, the elderly, and individuals with compromised immune systems. When fjöldónæmi is established, these individuals are indirectly protected by the immunity of the community around them. The decline or eradication of many infectious diseases, such as smallpox, is a testament to the effectiveness of fjöldónæmi achieved through widespread vaccination programs. Maintaining high vaccination rates is therefore essential to sustain this protective effect.