Herd immunity

Spread of contagious diseases in populations with different vaccination rates.

Herd immunity (from English herd immunity ) referred to in the epidemiology an indirect form of protection against infectious disease , which occurs when a high percentage of the population has already become immune - either through infection or by vaccination - is so spread capabilities of the pathogen decrease within the population as a whole. This indirectly results in increased protection for non-immune individuals as well. The term must be clearly distinguished from that of individual immunity .

Definitions

Herd immunity can be understood as:

• the proportion of a population (the “herd”) that is immune to certain communicable infectious diseases
• a parameter for the proportion of immunized persons in a population which, if exceeded, leads to a decrease in new infections in this population
• a pattern of immunity that protects a population from new infections
• the resistance of a group to attack by a disease
• the phenomenon that not every person in a population needs to be immunized to eradicate a disease

Moreover, the term herd immunity is in different ways with the term "herd effect" (English of. Herd effect ) associated:

• Herd effect is the indirect protection that is granted to the non-immunized part of the population, provided that those protected by infection or vaccination interrupt the chain of infection (also called community protection )
• Herd immunity is synonymous with the herd effect

The term herd immunity is thus used with different meanings. An understanding of the immunological phenomena associated with comprehensive immunization coverage of populations would help to improve the design of vaccination campaigns so that communicable infectious diseases can be better controlled, limited or eradicated not only in theory but also in reality. The definition of herd immunity as direct and herd effect as indirect protection is followed below.

Influencing variables

The phenomenon of herd immunity is complex. The simple (“crude”) threshold theory, ie the basic model for the interaction of transmission and immunity threshold, is “naive” because it is based on a number of assumptions that are not in reality and on factors that have not yet been established are sufficiently understood or quantifiable or still unknown.

The variables that have an impact on the actually achieved effectiveness (efficiency) of vaccinations include:

• Maternal immunity, with its effect on neonatal loan immunity and the immunogenicity of newborn vaccinations
• Age especially at the first vaccination
• Age with an effect on the type and frequency of contact with germ carriers
• Season with an effect on the type and frequency of contact with germ carriers and the ability of germs to multiply
• Duration of infectivity
• Duration and level of immunity
• Homogeneity of the population or of sub-populations in terms of type and frequency of contact and the transmission of infection between infected and non-immunized
• Homogeneity of the population or of sub-populations with regard to attitudes towards and compliant implementation of vaccinations
• Distribution of vaccines (random or targeted, homogeneous or inhomogeneous)
• Contact of sub-populations to the rest of the population (isolated or permeable)
• Distribution of immunity in sub-populations and the population
• individually impaired immune competence (for example due to immunodeficiency or vaccination failure )
• collective vaccination failure in sub-populations (e.g. due to hygienic conditions, nutritional status, ethnic-genetic differences)
• Type of immunity induced by vaccination (e.g. humoral or cellular, mucosal or systemic, oral or enteral, against the pathogen or against toxins)

The efficiency factor E introduced into the mathematical model to take these influencing variables into account is in turn based on assumptions as to how the aforementioned influencing variables affect quantitatively individually and in mutual interaction.

As a rule, factor E leads to an increase in the vaccination rate required in reality compared to the idealistic assumptions of the mathematical model. If, however, sub-populations are known that have a particularly high risk of infection, their targeted vaccination can lower the overall vaccine coverage required, i.e. lead to an average higher actual effectiveness of the vaccine campaign.

properties

The herd effect works like a firebreak in a fire, in that the chain of infection of a pathogen is interrupted or at least slowed down with vaccinations. As a result, the disease can no longer spread epidemically if the pathogens are only transmitted between people ( anthroponosis ). Herd immunity cannot be achieved against pathogens that survive outside their host, for example the bacterium Clostridium tetani , which occurs in the soil, which triggers tetanus or the TBE virus transmitted by ticks . Protection here is only offered by individual prophylaxis.

Above a certain threshold of immunity in a population, the population of the pathogen collapses due to a lack of reproduction and the disease can no longer circulate in this population. This threshold value is essentially dependent on the basic reproduction number of the respective pathogen. If the pathogen has refuge areas in other populations, it can no longer infect from these refuge areas from populations with sufficient herd immunity.

Minimum proportion of immunized patients required for the herd effect
for a selection of diseases that can be prevented by vaccination
(mathematical model for ideal conditions)

illness	Transmission path	R 0	Minimum proportion of immunized
measles	Droplet infection	12-18	83-94%
mumps	Droplet infection	4-7	75-86%
polio	fecal-oral infection	5-7	80-86%
rubella	Droplet infection	5-7	80-85%
smallpox	Droplet infection	6-7	83-85%
The base reproduction number R 0 indicates how many other people are infected on average by an infected person if the population or subpopulation surrounding them is neither protected by vaccination nor by previous infection.

The basic model

The minimum vaccination coverage required for a herd effect under ideal circumstances is given by:

${\displaystyle \operatorname {HI} _{\min }=1-{\frac {1}{R_{0}}}={\frac {R_{0}-1}{R_{0}}}}$

and in percentages:  

${\displaystyle \operatorname {HI} _{\min }=100-1/R_{0}\cdot 100}$.

  Here is is the basic reproduction number. The value is also called herd immunity threshold ( english herd immunity threshold , short- HIT ), respectively. ${\displaystyle R_{0}}$ ${\displaystyle \operatorname {HI} _{\min }}$

The herd immunity threshold (HIT) listed in the table is not identical to the minimum required vaccination rate:

Efficiency factor E

In contrast to the basic model, the extent of herd immunity and herd effect that can actually be achieved depends on the aforementioned influencing factors. Therefore, in reality, higher vaccination rates are usually required for a herd effect than under ideal conditions. To take this into account, the equation is expanded to include the quantity E, the factor of effectiveness ( for efficiency): ${\displaystyle E}$

${\displaystyle D_{\min }={\frac {\operatorname {HI} _{\min }}{E}}={\frac {1-{\frac {1}{R_{0}}}}{E}}}$

Thus, the minimum vaccination coverage required is usually higher than the minimum required herd immunity (HIT) listed in the table. Both an increase in the coverage rate and an increase in the effectiveness of the vaccination make it easier to achieve a herd effect. The favorable case in which the minimum necessary vaccination rate required on average (for a total population) is lower than the minimum necessary herd immunity listed in the table (for the total population) rarely occurs, for example in the case of targeted vaccination of sub-populations with a particularly high risk infection and / or transmission. Herd immunity is not a static phenomenon. For example, vaccinations with waning immunity over time require booster vaccinations to maintain a herd effect.

Protection of special groups of people

People who cannot be vaccinated effectively

Herd immunity is of particular importance for people who cannot be vaccinated with sufficient effectiveness, for example people with immunosuppression (disease of the immune system such as HIV infection, lymphoma , bone marrow cancer or leukemia ; chemotherapy or radiation therapy ; taking immunosuppressants after organ transplantation). In addition, contraindications can contribute to a lack of immunity. The lack of immunity in these groups of people can sometimes also lead to more severe disease processes, which can be avoided by a herd effect.

People who cannot be vaccinated yet

For example, side effects (especially from vaccines with live pathogens) or ineffectiveness due to passive immunity from the mother's antibodies can be reasons for administering individual vaccines in newborns . Pregnant women who are not yet protected from rubella by infection or vaccination are not vaccinated against rubella with the currently common combination vaccines because they contain live pathogens whose harmlessness for the unborn is not guaranteed. The herd immunity of their contact persons serves to protect such pregnant women and their unborn babies.

Protection of other age groups

High vaccination coverage in one age group can also protect people in other age groups from developing this pathogen, such as unvaccinated infants and young children or the elderly.

Sexually transmitted diseases

High-risk behavior during sexual intercourse leads to a high transmission rate of the pathogens concerned, which makes the eradication of sexually transmitted diseases more difficult. With sexually transmitted diseases , the herd effect may extend from one sex to the other sex.

Eradication

Cow with Rinderpest (1982). The last known case was in Kenya in 2001. The disease was declared eradicated in 2011.

In the best case scenario, a disease can even be eradicated ( eradication ) by means of sufficiently high vaccination rates . In other words, the pathogen is no longer endemic. The eradication of infectious diseases is a goal of health policy . In the case of smallpox , this was achieved through a consistent, worldwide vaccination and control program, so that in 1980 the WHO declared the world to be free of smallpox . The same has now almost been achieved for polio globally - only a few countries are still considered endemic for polio viruses ( Nigeria , India , Pakistan , Afghanistan ). However, as vaccination efforts decline in neighboring countries, there are repeated outbreaks of poliomyelitis through re-imports, most recently in 2006 in Namibia . The global elimination of measles, also set as a goal by the WHO, has so far only been achieved on the continents of America and Australia as well as in Scandinavia, as the vaccination rates are too low in the rest of the world. As a result, local measles epidemics break out again and again, including in Germany, for example, the regionally limited measles epidemics in Hesse, Bavaria, Baden-Württemberg and North Rhine-Westphalia, including serious complications and deaths in 2005/2006.

Negative epidemiological effects

As a rule, high vaccination coverage results in herd immunity and a herd effect. Large populations around the world have been protected from dangerous communicable infectious diseases. However, the findings of the pharmacologist Gustav Kuschinsky also apply to vaccines : "A drug that is claimed to have no side effects is strongly suspected of not having any main effect." According to the WHO, side effects of vaccinations should be specifically recorded and systematically classified become. In addition to individual side effects for some vaccinated persons (e.g. pain at the injection site, allergy to components of the vaccine), side effects of an epidemiological nature are also known. The Robert Koch Institute describes them as "undesirable negative effects of a vaccination strategy on the population level" and has declared their education to be one of its tasks. Examples of the negative epidemiological effects of vaccination at the population level include:

• Changes to prevailing serotypes (serotype replacement). The consequence can be, for example, that the effectiveness of previous vaccines decreases.
• Age shifts in the burden of disease. This can, for example, increase the likelihood and / or severity of complications from the infectious disease.
• Reverse mutation of attenuated live vaccines. It can lead to the formation of human pathogens in the vaccinated person and to the infection of third parties by these pathogens.
• Changes in predominant pathogen species. The type of pathogen that has become more common can, for example, have other living beings (“hosts”) as reservoirs in addition to humans and thus evade eradication.

Serotype replacement

Vaccinations can lead to a change in the relative and / or absolute frequency of the serotypes of the pathogen (vaccine-induced pathogen strain replacement). As far as the increase concerns serotypes that are not covered by the vaccine, the effect of the vaccination may be less than was to be expected based on the vaccine's effectiveness against the serotypes contained in the vaccine. A serotype replacement requires the replacement or expansion of the antigens in the vaccine. A major example of this is the pneumococcal vaccine .

Age shifts in the burden of disease

The herd effect can lead to an accumulation of people in sub-populations who are themselves immune to the pathogens of communicable infectious diseases, neither through infection nor vaccination. If germ carriers get into this group, for example through travel to endemic areas or immigration from such areas, and the herding effect is insufficient for this group, they risk developing an illness that was previously common as a childhood disease . Such age shifts have been documented for measles , for example . As people get older, measles is more difficult to detect, so that measles pneumonia , for example, is delayed and treated appropriately. In addition, measles occurs in newborns in the period up to the first vaccination (recommended between 11 and 14 months of age) when their mothers have been vaccinated against this pathogen than when the mothers had the measles as an infection because the loan immunity brought about via the placenta disappears faster after vaccination than after infection.

When it comes to mumps , the majority of infections in children run with little or no symptoms. In recent years, however, in Germany, as in many other European countries, more and more mumps outbreaks have occurred among adolescents and young adults. At this older age, the symptoms become more obvious, and in male patients they include the risk of sterility due to mumps orchitis .

With rubella , too, complications become more common as the sick person gets older. Also chickenpox and hepatitis A occur thanks of regulated vaccinations less frequently in childhood. If the herd effect diminishes, however, people who have not been vaccinated are often only infected at an advanced age. Then on average there are more frequent and more serious complications than in children.

After vaccination against whooping cough , antibodies pass through the placenta to the newborns. The pertussis antibodies, on the one hand, protect the newborns from life-threatening illnesses in the first two months and, on the other hand, (unlike occasional loan antibodies against rubella) do not hinder the development of their own antibodies in response to the newborn vaccinations that start in accordance with the rules from the end of the second month.

Reverse mutation of attenuated live vaccines

The most important example of this type of negative effects of population vaccinations is the reverse mutation of the pathogen in the oral polio vaccine (OPV) into a variant that is pathogenic to humans again , which is similar to the wild virus, and third party diseases caused by this virus ( circulating vaccine-derived poliovirus , cVDPV). The WHO therefore recommends that the “oral vaccination” with OPV be gradually switched over to the inactivated polio vaccine (IPV).

Increase in other pathogen species

As a result of the DTP vaccinations, the majority of reported diphtheria cases in Western Europe are caused by Corynebacterium ulcerans (skin diphtheria) and no longer by the previously classic diphtheria pathogen C. diphtheriae (pharynx diphtheria ). C. ulcerans is able to produce the diphtheria toxin and thus trigger the systemic symptoms of the disease. The usual diphtheria vaccine is also effective against C. ulcerans . But unlike C. diphtheriae, C. ulcerans has its reservoir in animals (including domestic animals), which makes its eradication more difficult.

Problems

Vaccination fatigue

Vaccination fatigue is a particular danger to herd immunity . Vaccination campaigns which do not achieve the necessary herd immunity can, under certain circumstances, increase the frequency of disease complications in non-vaccinated persons. If too low a proportion of the population is vaccinated, this “only” lowers the likelihood of infection among those who have not been vaccinated instead of preventing infection via herd immunity. This means that the infection, if it then occurs, often no longer occurs in childhood, which is more dangerous with some diseases such as mumps , rubella , polio , chickenpox . For example, an increase in rubella embryo fetopathy cases was reported in Greece in the early 1990s , after vaccination coverage was below 50% throughout the 1980s. For this reason, every vaccination campaign should not only aim to partially protect the population, but also ensure herd immunity. It is also important that those responsible for planning vaccination campaigns understand mathematical and epidemiological models of medicine. Education about the herd effect can reduce vaccination fatigue. In polio, eradication is delayed by political unrest and mistrust of modern medicine. A vaccination requirement could accelerate the eradication.

COVID-19 pandemic

Herd immunity in the course of the COVID-19 pandemic (caused by SARS-CoV-2 ) was mentioned and sometimes criticized in English-language and, occasionally, also in German media .

The British government under Boris Johnson initially advocated an - albeit uncontrolled (even without vaccination, which is / was not yet available) - herd immunity strategy to combat COVID-19 in the UK . But after warnings from science, she stopped doing it.

In an interview with Radio Eins , the German virologist Karin Mölling also suggested such an uncontrolled strategy. In an interview with KenFM on March 24, 2020, she said no when asked by Ken Jebsen whether she supported herd immunization. This is "today an ethically very dubious matter, because you basically let people run into the knife". It belongs to the "helplessness reactions because you have to try to do what", but "decision makers" should not "let people run into open knives". In an interview on April 8, 2020, forensic doctor Klaus Püschel considered a course of herd immunity to be inevitable.

In April 2020, it was still unclear in the ranks of the World Health Organization (WHO) whether herd immunity could even be developed with COVID-19. A complete immunity of former patients against a new infection lasting several months has not yet been adequately investigated, although animal studies and observed antibodies had suggested this. In South Korea, the virus was detected again using PCR in 91 patients who had already recovered; however, it remained unclear whether this corresponds to being sick again. This may have had various causes, including the always existing uncertainty with medical test results, including the detection methods of a Covid-19 disease . In another preliminary study from Shanghai, not all patients had developed the same high antibody levels, and in some the antibodies had not been detected in the laboratory.

Web links

• Herd immunity animation and interactive simulator. impfen-info.de
• Further information on the topic of the Scientific Academy for Preventive Medicine Graz
• How to achieve herd immunity dlf.de

Individual evidence

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