Vaccines are essential preventive tools against many severe infectious diseases. Below is an overview of the different vaccines types, with their advantages and shortcomings. A brief outline of classic and alternative delivery strategies is also provided, and examples of currently available and experimental vaccines are listed for each category and delivery method.
Vaccine types
Inactivated: infectious organisms are killed by heat or chemicals (e.g. formaldehyde) and used to induce a protective immune response against the disease they are responsible for. Due to complete inactivation of the agent, these vaccines are safe and rarely cause side effects. They are stable and do not require refrigeration. One disadvantage is that they may not be as immunogenic as other vaccine types because their structure can be modified by heat or chemical treatment. Examples of inactivated vaccines are the currently licensed poliovirus, hepatitis A and influenza vaccines.
Live attenuated: infectious organisms are genetically manipulated to remove part of their virulence, so they can be immunogenic without causing disease symptoms. They usually induce very strong immune responses, although they can still retain the ability to cause a milder form of the disease, especially in immunocompromised hosts (e.g. HIV patients). Successful live attenuated vaccines include the measles-mumps-rubella (MMR) formulation and the varicella (chickenpox) vaccine.
Conjugate: these vaccines are produced by linking an antigen to a carrier (usually a protein) by a chemical reaction known as covalent bond. This process is done so that poorly immunogenic vaccines (usually sugar components from bacteria) become more protective against infectious diseases. These vaccines are generally safe and effective, although they do not cover all bacterial serotypes as in the case of the pneumococcal 13-valent vaccine. Other current examples are the meningococcal and Hemophilus b conjugate vaccines.
Subunit: components such as proteins or sugars are isolated from microbes for use as vaccines. These antigens are extremely safe because they are only portions of pathogens and cause little side effects. One negative aspect is that their identification in the laboratory can take several months or years. The use of adjuvants is usually required for optimal efficiency. A subunit vaccine currently in use is the one against hepatitis B, based on a protein isolated from the viral envelope.
Recombinant and DNA: based on the use of genetic material of a microorganism, these two vaccine types are promising but currently only experimental. Research is ongoing to design formulations against several pathogens for which no vaccine is available, including HIV.
Vaccine delivery routes
Parenteral routes
Parenteral administration is a classic immunization route based on needle injection. Once a vaccine has been introduced in the body, it spreads in various organs inducing antibody responses and granting protection from disease.
- Intramuscular: currently licensed formulations such as Gardasil for HPV, hepatitis A, rabies and influenza vaccines are injected directly into the patient’s muscle.
- Subcutaneous: MMR, meningococcal, and varicella vaccines are delivered by injection in the layer of skin under the dermis.
Mucosal routes
Mucosal vaccination is an attractive alternative to defeat pathogens at their port of entry (e.g. nasal mucosa), preventing their spread to other organs. This strategy is convenient, not invasive and less expensive than parenteral routes. Nevertheless, challenges exist to develop this type of vaccines. Mucosal surfaces are rich in elements known as enzymes that could break down a vaccine before it diffuses in other body sites. Also, adverse reactions can occur, hence safety needs to be addressed.
- Oral: two currently licensed vaccines given orally are the Sabin oral polio and the rotavirus vaccines. Studies are being conducted to test oral vaccines against Salmonella.
- Nasal: the live FluMist against influenza is the only approved nasal vaccine. Other candidates are being tested to prevent diseases like tuberculosis and bacterial pneumonias.
- Vaginal and rectal vaccination routes are promising for the prevention of sexually transmitted diseases, but are currently only experimental.
Helpful definitions
Adjuvants: compounds that increase the efficacy of a vaccine by aiding stimulation of the immune system via several mechanisms.
Antibodies: proteins, from the family of immunoglobulins, released by cells of the immune system in response to vaccination or infection.
Antigens: substances that induce antibody production once introduced into the body. They can be either vaccines or infectious agents.
Immunogenic: capable of triggering an immune response.
Pathogens: microorganisms capable of inducing disease.
Vaccines: formulations that induce protective immune responses against infectious agents.
Virulence: degree of pathology caused by a microorganism.
References
- FDA approved vaccines
- National Institute of Allergy and Infectious Diseases on Vaccines
- Vaccines, 5th Edition. Expert Consult by Stanley A. Plotkin, MD, Walter A. Orenstein, MD and Paul A. Offit, MD
Damiana Chiavolini - I have written over ten articles published in peer-reviewed scientific journals, but my appetite for writing goes beyond microbiology, ...
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