Flockimmunitet

Flockimmunitet, the Danish term for herd immunity, refers to indirect protection from an infectious disease that occurs when a sufficient proportion of a population becomes immune, thereby reducing transmission. Immunity can arise from vaccination or from by prior infection. When transmission falls, even those who are not immune gain some protection because the pathogen has fewer opportunities to spread.

The herd immunity threshold depends on the pathogen’s basic reproduction number (R0) and contact patterns. A common rule of thumb is 1 − 1/R0. For diseases with high R0, such as measles, the threshold can be around 95%. For others it may be lower. In practice, the required level varies with population structure, waning immunity, and the emergence of variants or new strains.

Vaccination plays the central role in achieving flockimmunitet. Vaccines reduce susceptibility and, in some cases, transmission. High and equitable vaccination coverage is usually required to reach the threshold, and the effectiveness and duration of immunity influence the attainable level. Relying on natural infection to build herd immunity carries substantial health risks, including severe disease and death.

Limitations and uncertainties include waning immunity, immune escape by variants, and uneven vaccine uptake. Behavioral changes, seasonality, and demographic heterogeneity can shift or complicate thresholds. In some diseases, herd immunity may be difficult or undesirable to pursue as a sole strategy; public health goals often focus on reducing severe disease and transmission rather than achieving complete flockimmunitet.

Context: the concept is widely used in public health discussions in Danish-speaking regions. Its applicability varies by disease and over time, and strategies typically combine vaccination with surveillance, ventilation, and other measures to control spread.