How vaccines work

Immunization

There are two artificial methods to make an individual immune to a disease

Active immunization:  Administration of a vaccine, so that the patient actively mounts a protective immune response.

Passive immunization:  Individual acquires immunity through the transfer of antibodies formed by an immune individual or animal.

Vaccines:

The principle of vaccination is to induce a "primed" state in the vaccinated subject so that, following exposure to a pathogen, a rapid secondary immune response is generated leading to the accelerated elimination of the organism and protection from clinical disease. Success depends on the generation of memory T and B cells and the presence in the serum of neutralizing antibody.

Attributes of a good vaccine

1. Ability to elicit the appropriate immune response for the particular pathogen:

Tuberculosis - cell mediated response

Most bacterial and viral infections - antibody

2. Long term protection
    ideally life-long
3. Safety
    vaccine itself should not cause disease
4. Stable
    retain immunogenicity, despite adverse storage conditions prior to administration
5. Inexpensive

How vaccines work:

A vaccine contains material intended to induce an immune response, and this may involve both B cells (which develop into antibody-producing cells) and T cells (responsible for cell-mediated immunity).

The purpose of most viral vaccines is to induce long-term immunity against the virus by establishing immunological memory.

Types of vaccines:

Live/attenuated virus vaccines

Killed-virus vaccines

Recombinant virus vaccines

Capsid and subunit vaccines

DNA vaccines

Edible vaccines

Live Vaccines

1. Live attenuated organisms

Organisms whose virulence has been artificially reduced by in vitro culture under adverse conditions, such as reduced temperature. This results in the selection of mutants which replicate poorly in the human host and are therefore of reduced virulence.  Replication of the vaccine strain in the host reproduces many of the features of wild type infection, without causing clinical disease.  Most successful viral vaccines belong to this group.

The immune response is usually good - when the virus replicates in the host cells, both antibody as well as cell mediated immune responses are generated and immunity is generally long lived.  Often, only a single dose is needed to induce long term immunity.

Serial passage is a blind procedure and the results cannot be predicted.

Specific mutants can be produced, either as deletions of regions of the genome or as site-specific changes, such that the properties of the putative vaccine can be customized.Eg: Polio vaccine - (Salk) Intramuscular; Polio (Sabin)-Oral, MMR vaccine; Rotavirus vaccine.

Advantages:

Both humoral and cell-mediated immune responses are stimulated.

Immunity develops after one or at most three exposures and usually lasts many years.

Disadvantages:

Reversion to virulence by back mutation is a problem (Sabin type 3 poliovirus vaccine)

Preserved with refrigeration, this problem can be partially alleviated by freeze-drying (lyophilizing), requires reliable source of sterile water for rehydration.

Derivation of attenuated poliovirus strain (Sabin type 1) from wild-type poliovirus strain

Schematic Diagram of Development of Attenuated Cell Strain

2. Heterologous vaccines

Closely related organisms of lesser virulence, which share many antigens with the virulent organism. The vaccine strain replicates in the host and induces an immune response that cross reacts with antigens of the virulent organism. The most famous example of this type of vaccine is vaccinia virus:   Both cowpox virus and vaccinia virus are closely related to variola virus, the causitive agent of smallpox. Widespread use of vaccinia virus as a vaccine has lead to the world-wide eradication of smallpox.

Killed (inactivated) vaccines:

When safe live vaccines are not available, either because attenuated strains have not been developed or else because reversion to wild type occurs too readily, it may be possible to use an inactivated  preparation of the virulent organism to immunize the host.

The organism is propagated in bulk, in vitro, and inactivated with either beta-propiolactone or formaldehyde. These vaccines are not infectious and are therefore relatively safe. However, they are usually of lower immunogenicity and multiple doses may be needed to induce immunity. In addition, they are usually expensive to prepare.

Subcellular fractions

When protective immunity is known to be directed against only one or two proteins of an organism, it may be possible to use a purified preparation of these proteins as a vaccine. The organism is grown in bulk and inactivated, and then the protein of interest is purified and concentrated from the culture suspension. These vaccines are safe and fewer local reactions occur at the injection site. However, the same disadvantages of poor immunogenicity and the need for multiple boosters applies.

Recombinant proteins

Immunogenic proteins of virulent organisms may be synthesized artificially by introducing the gene coding for the protein into an expression vector, such as E-coli or yeasts. The protein of interest can be extracted from lysates of the expression vector, then concentrated and purified for use as a vaccine. The only example of such a vaccine, in current use, is the hepatitis B vaccine.

Formaldehyde is commonly used to inactivate microbes by cross-linking their proteins and nucleic acids

Recognized as exogenous antigens and stimulate a TH2 response that promotes antibody-mediated immunity.

An advantage of the killed-virus vaccines is absence of the virus’s capacity to revert to virulence.

Killed-virus vaccines can be stored more cheaply than can live-virus vaccines.

Disadvantages are- multiple rounds of immunization are generally required

Vaccination does not result in complete immunity because an active infection does not occur

Immunity is not prolonged, eg. influenza vaccine, polio vaccine, hepatitis A vaccine, and rabies vaccine.

Subunit Vaccines:

Immune response can be stimulated by one or a set of viral proteins.

This was first demonstrated by hepatitis B and influenza vaccines

These can be a lot safer than attenuated or inactivated vaccines

The subunits included are determined by identifying which proteins the antibodies recognize.

Subunit vaccines are composed solely of purified protein & can be delivered to body by means of a non-pathogenic virus, bacteria, etc.

Desired immune response is most often directed against surface capsid or envelope protein of a pathogenic virus.

This protein by itself could be used as a vaccine if it were properly presented to the immune system.

A subunit vaccine can be prepared by purification of the protein subunit from the viral particle, or by recombinant DNA cloning and expression of the viral protein in a suitable host cell. eg: Hepatitis B virus surface antigen (HBsAg) as Hepatitis vaccine.

Vaccines for human papilloma virus (HPV 6, 11, 16, and 18) are known to be associated with cervical carcinomas.

Vaccines for influenza A and B

RECOMBINANT Vaccines:

3. Live recombinant vaccines

It is possible, using genetic engineering, to introduce a gene coding for an immunogenic protein from one organism into the genome of another (such as vaccinia virus). The organism expressing a foreign gene is called a recombinant. Following injection into the subject, the recombinant organism will replicate and express sufficient amounts of the foreign protein to induce a specific immune response to the protein.

Attributes

Good immune response, Both Cell Mediated Immunity and antibody responses, Immunity is long lived, Single dose, Safety, Danger of reversion to virulence, or severe disease in immunocompromised, Stability, Organisms in the vaccine must remain viable in order to infect and replicate in the host, Vaccine preparations are therefore very sensitive to adverse storage conditions, Maintenance of the cold chain is very important, Expense: Cheap to prepare.

Recombinant virus vaccines

Genetic recombinations are introduced into the genome of another virus, which is avirulent.

Protective immunity can be achieved by proteins expressed by recombinant proteins.

The carrier virus should be absolutely avirulent.

The carrier virus would be able to replicate, and able to generate protein or proteins.

carrier viruses examples are poxviruses, herpes viruses, adenoviruses and vaccinia virus

eg: Vaccine for Newcastle disease of chickens has been generated using recombinant cucumber mosaic virus of plants.

HBsAg, rabies, HSV and other viruses have been expressed in vaccinia.

DNA Vaccines

DNA vaccines are at present experimental, but hold promise for future therapy since they will evoke both humoral and cell-mediated immunity, without the dangers associated with live virus vaccines.

The gene for an antigenic determinant of a pathogenic organism is inserted into a plasmid.  This genetically engineered plasmid comprises the DNA vaccine which is then injected into the host.  Within the host cells, the foreign gene can be expressed (transcribed and translated) from the plasmid DNA, and if sufficient amounts of the foreign protein are produced, they will elicit an immune response.

in recent years a new type of vaccine, created from an infectious agent's DNA called DNA vaccination, has been developed. It works by insertion (and expression, triggering immune system recognition) into human or animal cells, of viral or bacterial DNA. These cells then develop immunity against an infectious agent, without the effects other parts of a weakened agent's DNA might have. As of 2006, DNA vaccination is still experimental, but shows some promising results.

Production of a DNA vaccine:  The virus protein gene is inserted into a plasmid, which is then cloned in bacteria. The plasmid is extracted from the bacterial cells, purified and incorporated into a vaccine.

DNA vaccines Vs. Traditional vaccines:

DNA vaccines: Uses only the DNA from infectious organisms, Avoid the risk of using actual infectious organism, Provide both Humoral & Cell mediated immunity, Refrigeration is not required.

Traditional vaccines: Uses weakened or killed form of infectious organism, Create possible risk of the vaccine being fatal, Provide primarily Humoral immunity, Usually requires Refrigeration.

Edible vaccines

Another approach is to express the antigenic protein or antigenic portion of a protein in a form that could be ingested and still generate protective immunity.

Such an antigen would need to be able to survive the digestive system and be assimilated by antigen-presenting cells.

Currently, efforts are underway to generate transgenic plants in which antigenic peptides are incorporated into cereal grains, legumes, and even potatoes, so that food sources could be made available to provide protection against one or another major human or animal disease.

Various foods under study are banana, potato, tomato, lettuce, rice, etc. Edible vaccines are currently being developed for a number of human and animal diseases, including measles, foot and mouth disease and hepatitis B, C and E.