Herd immunity

Herd immunity (or community immunity) occurs when a high percentage of the community is immune to a disease (through vaccination and/or prior illness), making the spread of this disease from person to person unlikely. Even individuals not vaccinated (such as newborns and the immunocompromised) are offered some protection because the disease has little opportunity to spread within the community.

Vaccines prevent many dangerous and deadly diseases. However, there are certain groups of people who cannot get vaccinated and are vulnerable to disease: babies, pregnant women, and immunocompromised people, such as those receiving chemotherapy or organ transplants. For example, the earliest a baby can receive their first pertussis or whooping cough vaccine is at six weeks, and the earliest a child can receive their first measles vaccine is at nine month, making them vulnerable to these diseases.

Herd immunity protects the most vulnerable members of our population. If enough people are vaccinated against dangerous diseases, those who are susceptible and cannot get vaccinated are protected because the germ will not be able to find those susceptible individuals.

Why are there still outbreaks of vaccine-preventable diseases?

There are a number of reasons why people are unprotected: some protection from vaccines “wanes” or “fades” after a period of time. 

Some people don’t receive all of the shots that they should to be completely protected. For example you need two measles, mumps, and rubella (MMR) injections to be adequately protected. Some people may only receive one and mistakenly believe they are protected. 

Some people may object because of religious reasons, and others are fearful of potential side effects or are skeptical about the benefits of vaccines.

One of the drawbacks of herd immunity is that people who have the same beliefs about vaccinations frequently live in the same neighborhood, go to the same school, or attend the same religious services, so there could be potentially large groups of unvaccinated people close together. Once the percentage of vaccinated individuals in a population drops below the herd immunity threshold, an exposure to a contagious disease could spread very quickly throughout the community.

Immunity is the biological state of being able to resist disease: the primary objective of vaccination is to induce an immunological memory against specific diseases, so that if exposure to a disease-causing pathogen occurs, the immune response will neutralise the infection before disease can occur.

 Immune recognition

One of the primary ways in which the immune system achieves elimination of pathogens and other unwanted foreign material is through a ‘self’ tag. Each cell in the body is equipped with a type of molecule that identifies the individual from any other, much like a barcode. Pathogens not only lack a ‘self’ tag, they also contain a range of material termed ‘virulence factors’ that the immune system recognises as danger signals.

Antigens (antibody generators) are the drivers of an immune response. Antigens are usually part of a foreign protein or glycoprotein; molecular shapes that the immune system recognises as foreign and trigger an adaptive immune response. While some vaccines contain the entire weakened or attenuated organism (such as measles, mumps and rubella vaccines), increasingly vaccines now contain purified antigens (as in acellular pertussis, HPV or pneumococcal vaccines).

The first process that occurs when a foreign antigen, such as a vaccine antigen, is introduced to the body is the recognition that the antigen is non-self. The antigen is taken up at the local site (such as the injection site) by professional phagocytic cells called antigen-presenting cells; for example, macrophages and dendritic cells. Once inside the antigen- presenting cells, degradation of the foreign protein (or microbe) occurs and tiny fragments are carried to the cell surface and displayed along with a ‘self’ tag molecule. These antigen-presenting cells then make their way through the lymph to the local lymph node where the adaptive immune response is initiated.

Induction of the adaptive immune response.

The response that occurs the first time an antigen is ‘seen’ by the immune system is called the primary immune response.

The adaptive immune response occurs in lymphoid tissue, primarily the lymph nodes, of which there are 500–600 distributed throughout the body, including the spleen.

The adaptive immune response to most vaccines occurs at the draining lymph node proximal to the site of injection. The spleen and lymph nodes are densely populated with important effector lymphocytes of the immune response: the T-cells and B-cells. The lymph that flows through the nodes brings with it the vaccine antigen that has been captured at the injection site by the specialised antigen-presenting cells. Once in the lymph node the vaccine antigen, in combination with the cell that has carried it there, comes into contact with the specific T-cells and B-cells.

Among the trillions of specific T and B lymphocytes (~1016 possibilities) there (usually) exists a match for the antigen. The process that occurs once these cells recognise each other is the primary immune response and it matures over a period of four to six weeks.

An early outcome of the interaction between these antigen-presenting cells and T and B lymphocytes is the production of antibody-producing B-cells. Antibody can be measured in the blood as soon as 4–7 days, but is usually more effectively measured weeks to months later

 

Development of immune memory and the secondary response.

The response that occurs the second time an antigen is ‘seen’ by the immune system is called the secondary immune response.

Following the primary immune response, a reaction occurs within the lymph node. Over a period of around two months, cells that are less specific for the specific antigen are deleted, and those that are highly specific are retained and divide. During this time immunological memory cells also develop.

The next time the same antigen is introduced, either as a pathogen component or as a further dose of vaccine subunit, the immunological memory cells will recognise it and begin to proliferate. Highly specific antibody (primarily of the IgG subtype, but also IgA) is rapidly produced in large amounts. The lag phase is much shorter than the primary immune response (see Figure 1.1), just 1–4 days; the antibody peaks very quickly and lasts much longer.

Acquisition of adaptive immunity

Specific antibody can be acquired either naturally via infection or ‘artificially’ using vaccines by teaching the immune system to respond to specific parts of the potential pathogenic antigens. This is termed adaptive or learnt immunity.

Naturally acquired immunity.

Naturally acquired immunity occurs either actively by experiencing the infection or passively through the transfer of maternal antibodies from mother to fetus or infant (transplacentally or in breastmilk).

Artificially acquired immunity.

Artificially acquired immunity occurs either actively through vaccination or passively through administration of immunoglobulin (IG) 

While actively acquired immunity lasts from years to life, passively acquired immunity lasts from weeks to months as the transferred antibodies decay and are not renewed.

Innate immunity

Most infectious microbes (also known as micro-organisms) are prevented from entering the body by barriers such as skin, mucosa, cilia and a range of anti-microbial enzymes. Any microbes that breach these surface barriers are then attacked by other components of the innate immune system, such as polymorphonuclear leucocytes (neutrophils), macrophages and complement.

This non-specific immune response termed ‘innate’ is robotic and does not involve learnt or adaptive mechanisms. The cells and proteins of the innate immune system are able to recognise common microbial fragments and can kill microbes without the need for prior exposure. The cells of the innate immune system also interact with the cells of the adaptive immune system (eg, lymphocytes) to induce a cascade of events that results in the development of adaptive immunity and immune memory.

By protecting individuals, vaccination can also protect the wider community. This herd immunity occurs when the vaccine coverage is high, meaning an infectious case is unlikely to encounter susceptible contacts, so transmission stops.

When a vaccine is able to prevent carriage and transmission of a human- only pathogen such as polio virus, measles virus or Streptococcus pneumoniae, the whole population benefits, and these agents can be reduced and even eliminated. This phenomenon, called herd or community immunity, can prevent infections spreading and therefore protect vulnerable members of the population, such as the very young, very old, or those with underlying conditions that increase their risk from infectious diseases (immunocompromised). These individuals may not themselves be able to receive some vaccines (eg, live vaccines) or may not mount an effective immune response to other vaccines.The population benefits depend on the disease itself and the nature of the vaccine. 

Corona virus and herd immunity.

For herd immunity to kick in, about 60%, (some say 70%) of the population would need to contract the virus.

Reproduction number (R0) and herd immunity threshold (H)

A measure of the infectiousness of a disease is the basic reproduction number (R0),

The herd immunity threshold (H) is the proportion of immune individuals in a population that must be exceeded to prevent disease transmission.