Herd immunity

Herd immunity ( from the English. Herd immunity ) refers to the effect, in which the generated or acquired by infection by vaccination immunity against a pathogen within a population (the " flock" ) is so common that are protected in the population of non- immune individuals because the pathogen can not spread. Particular importance has the herd immunity for persons who can not be vaccinated, such as people with immunosuppression ( immune system disease; organ transplantation ), or newborns who have not yet been vaccinated.

Herd immunity acts like a firebreak in a fire, by the chain of infection of a pathogen is interrupted by vaccination or at least slowed down. As a result, the disease, the pathogen can not spread epidemic, so far only be transmitted between humans ( anthroponosis ). For pathogens with refuges, such as the occurring in the soil bacterium Clostridium tetani as causing tetanus or the tick- transmitted to other mammals TBE virus, herd immunity can not be achieved. Protection from these diseases is only the individual prophylaxis.

Beyond a certain threshold vaccination rate in a population, the population of the pathogen breaks down due to lack of replication and the disease can no longer circulate in this population. This threshold is essentially dependent on the basic reproduction rate of the respective pathogen. Although the pathogen has safe havens in other populations, it can not manifest itself only occurs sporadically and locally by reexports.

In the best case, a disease so even eradicated by sufficiently high vaccination rates in a population, ie the pathogen is not endemic. In the pox exactly this could be achieved by a consistent, global vaccination and control program, so that in 1980 the world could be explained by the WHO for free of smallpox. The same was for polio globally now reached almost - now only a few countries are still considered endemic for polio viruses ( Nigeria, India, Pakistan, Afghanistan). In declining vaccination efforts in neighboring countries but it always comes back to outbreaks of poliomyelitis by re-imports, most recently in 2006 in Namibia. The global elimination of measles, also specified by the WHO as a target, but until now could only be achieved on the continents of America and Australia and in Scandinavia, as in the rest of the world, the coverage rates are too low. As a result, repeatedly breaking local measles epidemics, for example, also in Germany, the limited regional measles epidemics in Hesse, Bavaria, Baden- Württemberg and North Rhine -Westphalia, including serious complications and deaths in the years 2005/2006.

A threat to herd immunity is particularly vaccination fatigue dar. vaccination campaigns that do not achieve the necessary herd immunity, under certain circumstances, the incidence of disease complications may increase in non- vaccinees. If too small a proportion of the population vaccinated, lowers this "only" the probability of transmission in non- vaccinated, rather than an infection on herd immunity to prevent. This means that the infection, if it takes place then, often no longer occurs in childhood, which is dangerous in some diseases, such as mumps, rubella, polio, chickenpox. For example, in Greece reported in the early 90s by a growing number of cases of congenital rubella syndrome after the vaccination rate in the entire 1980s, less than 50%.

For this reason, any vaccination campaign should aim not only partial protection of the population, but also to ensure herd immunity. Also it is important that those responsible which vaccination campaigns plan and understand mathematical epidemiological models of medicine.