Vaccine types

Also known as: Immunization

Definition

A vaccine is a suspension of immunogens (molecules that produce an immune response or stimulate production of antibodies) such as weakened or dead pathogenic (disease-causing) cells or cellular components. The act of administering a vaccine, or immunization, is called vaccination. Persons who receive a vaccine are considered immunized against a particular pathogen. Vaccines may contain a pathogen, suspending fluid, adjuvants, excipients, and preservatives.

Several types of vaccines are given to humans. These types include live attenuated, inactivated, component or subunit, toxoid, conjugate, deoxyribonucleic acid (DNA), recombinant vector, and messenger RNA vaccines, also called mRNA vaccines. Live attenuated vaccines contain living but altered bacteria or viruses that do not cause disease. Inactivated or killed vaccines contain killed bacteria or inactivated viruses that do not cause disease. Component or subunit vaccines contain parts of the whole bacteria or viruses. Toxoid vaccines contain toxins (or poisons) produced by the pathogen that have been made harmless. Conjugate vaccines allow the immune system to recognize certain bacteria disguised by a polysaccharide outer coating and therefore respond. DNA, mRNA, and recombinant vector vaccines all use genetic material to stimulate an antibody response.

Some vaccines are combinations of pathogens for different diseases, such as that for measles, mumps, and rubella (or MMR vaccine). Most vaccines are administered by injection into the muscle (intramuscular); however, some may be given into the skin (subcutaneous), by mouth, or into the nose (intranasal).

Active immunity is classified as natural (after pathogen exposure and infection) or acquired (after vaccination). Passive immunity is also classified as natural (across the placenta during pregnancy) or acquired (injection of antibodies or immunoglobulins pooled from several donors). Immunoglobulins are prepared antibodies that are given to a person who has already been infected or who is at risk of acquiring an infection, thereby providing passive immunization. In this case, the immune system does not need to produce antibodies protecting the body.

Herd immunity occurs when most of, but not all, the people in a given population are immune to a pathogen. If there is an outbreak or exposure to a pathogen, those who are immune will sometimes naturally protect those who are not immune from getting the disease; however, those who are not immune are still more likely to get the disease and spread it to others.

Mechanisms of Action

A vaccine is given to intentionally expose the immune system to a pathogen in a safe, controlled manner, so that the immune system can react and develop antibodies to that pathogen or antigen. Antibodies are large proteins that help fight infection and control disease. Many antibodies disappear after destroying the invading antigens, but the cells involved in antibody production remain and become memory cells. Memory cells “remember” the original antigen and then defend against it if the antigen attempts to reinfect a person. This protection is called immunity. Therefore, after sufficient antibodies have been developed, the immune system that is re-exposed to that pathogen will react within minutes to hours; the pathogen will be destroyed before a full-fledged infection and organ damage can occur. However, antibodies are only produced regularly by the body seven to fourteen days after the vaccine is given. Until this time, the vaccine does not offer full protection. B cells are a type of lymphocyte (white blood cell) that makes antibodies. B cells use antibodies to identify, inactivate, and help destroy these pathogens.

Vaccines, which provide protection from the disease without the serious symptoms, have a high effectiveness rate (usually 95 to 99 percent). Vaccine failure, meaning that the vaccine administration did not result in antibody production, is uncommon. Several factors can lead to vaccine failure, including having a compromised immune system, inadequate storage or administration of the vaccine, or, most commonly, vaccines fail when immune globulin is not used simultaneously. The immune response to a pathogen may decrease over time, so vaccines known as boosters are sometimes given to restore antibodies. Protective immunity lasts longer with boosters.

A suspending fluid (such as sterile water or saline) is needed to allow the vaccine to be administered. Preservatives and stabilizers, such as albumin, phenols, and glycine, keep the vaccine from being changed. Adjuvants, or enhancers, help the vaccine work. Adjuvants help promote an earlier, more potent response and a more persistent immune response to the vaccine. Antibiotics prevent the growth of bacteria during production and storage of the vaccine. Eggs are used to grow the pathogen, and egg protein is found in influenza and yellow fever vaccines. Formaldehyde is used to inactivate bacterial products for toxoid vaccines and to kill unwanted viruses and bacteria that might contaminate the vaccine during production. Monosodium glutamate and 2-phenoxy-ethanol are preservatives that help the vaccine remain unchanged during the vaccine’s exposure to heat, light, acidity, or humidity. Thimerosal is a mercury-containing preservative that helps prevent contamination and growth of bacteria.

Most vaccines are given to prevent disease and are effective only if administered to the person before he or she is exposed to the pathogen or disease; most vaccines must be given by a certain age to ensure effectiveness. Also, most vaccine-preventable diseases can cause serious or life-threatening infections in infants and young children. For example, exposure and infection with polio can occur at a very young age and can cause paralysis, so the vaccine should be given to infants as soon as possible. Immunity to some pathogens can be transferred from a pregnant woman to her fetus, but this immunity wanes once the newborn is older than six months of age. Breastfeeding can also help extend immunity to some diseases, but even this is limited.

Certain vaccines (such as pneumococcal or hepatitis B vaccines) are given once in a lifetime, unless a booster is needed. The seasonal influenza vaccine, however, is given annually because hundreds of influenza-like viruses exist; also, the seasonal variations or types of virus that are prevalent change every year. Vaccination schedules have been developed for children, adolescents, and adults that indicate when these persons should receive doses of required vaccinations or boosters.

Vaccine Types

The selection of the type of vaccine depends on fundamental information or factors about the pathogen. These factors include how the pathogen infects cells and how the immune system responds to it. Practical considerations include the regions of the world where the vaccine would be used. Pros and cons are associated with each type of vaccine.

Live attenuated vaccines. Live attenuated vaccines are usually created from the naturally occurring pathogen. The pathogen’s ability to cause serious infection is attenuated, or weakened, by manipulating the virus or bacteria in a laboratory environment, but these vaccines can still induce antibody production or a protective immune response. Attenuation of the pathogen usually is done by “passing” or growing the virus or bacteria from culture to culture before it is formulated into a vaccine. Live attenuated vaccines elicit strong cellular and antibody responses and often confer lifelong immunity with only one or two doses. Not everyone can safely receive live attenuated vaccines, however. People with weakened immune systems cannot be given live vaccines because of the risk they will develop disease symptoms.

These types of vaccines usually need to be refrigerated to stay potent. Proper storage then becomes critical in maintaining vaccine efficacy. Examples of live attenuated vaccines include measles, mumps, and rubella (MMR vaccine), oral polio vaccine (OPV), the nasal form of the influenza (flu) vaccine, and the varicella vaccine (chickenpox vaccine).

Inactivated vaccines. Inactivated vaccines contain a killed pathogen that cannot cause the disease but can stimulate antibody production. Pathogens can be inactivated with chemicals such as formaldehyde. Inactivated vaccines are more stable and safer than live vaccines. These vaccines usually do not require refrigeration and are easily stored and transported in freeze-dried form, making them useful in situations requiring long transportation or with less-developed medical infrastructure. Most inactivated vaccines, however, produce a weaker immune response than do live vaccines. Several additional doses or booster shots, therefore, are needed to maintain immunity. Examples of inactivated vaccines include inactivated polio vaccine (IPV) and inactivated (injectable form) influenza vaccine.

Component or subunit vaccines. Component or subunit vaccines are made by using only parts of the pathogen. These vaccines cannot cause disease, but they can stimulate the body to produce an immune response against the disease. Component vaccines contain only the essential antigens, but not all the other molecules, of the pathogen, so the chance of an adverse reaction to the vaccine is lessened.

These vaccines can contain anywhere from one to twenty or more antigens. Identifying what antigens best stimulate the immune system can be a tricky, time-consuming process. A recombinant component vaccine has been created for the hepatitis B virus. Hepatitis B genes that code for important antigens were inserted into common baker’s yeast. The yeast then produced the antigens, which were collected and purified for use in the vaccine.

A conjugate vaccine is another type of component vaccine that has been developed for bacterium that possesses an outer coating of sugar molecules called polysaccharides. The polysaccharide coating disguises the internal antigens of the bacterium so that the immune system does not recognize or respond to it. Vaccines help the immune system link the polysaccharide coating to the bacterium and, therefore, allow antibodies to produce immunity to that pathogen. Examples of component vaccines include Haemophilus influenzae type B (Hib) vaccine, hepatitis B (Hep B) vaccine, hepatitis A (Hep A) vaccine, and pneumococcal conjugate vaccine.

Toxoid vaccines. Toxoid vaccines are made by treating the toxin produced by the pathogen with heat or chemicals, such as formalin (a solution of formaldehyde and sterilized water). For pathogens that secrete toxins or harmful chemicals, a toxoid vaccine may be used when the toxoid is the main cause of illness. Toxins are inactivated and do not produce disease. Detoxified toxins are called toxoids. After vaccination with a toxoid vaccine, the immune system produces antibodies that block the toxin. Examples of toxoid vaccines include those against diphtheria and tetanus.

DNA and mRNA vaccines. DNA and mRNA vaccines contain the genes that code for antigens. This requires that the genes from the pathogen be analyzed. DNA and mRNA vaccines stimulate an immune response to the free-floating antigen secreted by cells and stimulate a response against the antigens displayed on cell surfaces. These vaccines contain copies of a few of the pathogen’s genes, so the vaccine does not cause disease.

DNA vaccines are relatively easy and inexpensive to design and produce. Naked DNA vaccines, which consist of DNA that is administered directly into the body, could be mixed with molecules that facilitate its uptake by the body’s cells. Some of the naked DNA vaccines researched and subjected to clinical trials as of 2022 included Venezuelan equine encephalitis virus, Ebola virus, Zika virus, Middle East respiratory syndrome coronavirus (MERS-CoV), influenza, HIV, human hepatitis virus C, herpesvirus, and cytomegalovirus.

Messenger RNA vaccines were first developed and authorized for use in the United States to protect against the virus caused by the coronavirus disease 2019, also known as COVID-19. The COVID-19 pandemic led to an unprecedented movement to develop a vaccine against the disease. Scientists and pharmaceutical companies around the world worked to accelerate the usual timeline for such a major undertaking. In November 2020, US-based drug manufacturers Moderna and Pfizer announced promising results from phase 3 trials, and both companies requested emergency use authorization (EUA) from the US Food and Drug Administration (FDA) for their two-dose vaccines. The FDA authorized the distribution of both the Pfizer-BioNTech vaccine, announced December 11, with the first vaccinations taking place on December 14, and the Moderna vaccine, announced December 18. Both vaccines require refrigerated storage at a very low temperature and require two shots, separated by 21 to 28 days, as well as boosters. In February 2023, around 400 million Pfizer vaccines were administered in the US, and more than 251 million Moderna vaccines. Booster numbers were much lower, with 34 million Pfizer and 19.5 million Moderna.

Recombinant vector vaccines. Recombinant vector vaccines, also called viral vector vaccines, use an attenuated pathogen to introduce DNA to cells of the body. A vector in this case is a harmless virus or bacterium used as a carrier. Certain harmless or attenuated viruses are used to carry portions of the genetic material from other microbes. The carrier viruses then ferry the microbial DNA to cells and display the antigens of the pathogen on the cell’s surface. The harmless organism mimics a pathogen and provokes an immune response. Recombinant vector vaccines closely mimic a natural infection, effectively stimulating the immune system. Recombinant vector vaccines for human immunodeficiency virus (HIV), rabies, and measles were being investigated in 2022. In February 2021, a single-dose vaccine using viral vector technology and developed by Janssen Pharmaceutical Companies of Johnson & Johnson was found to be safe and effective to prevent COVID-19 in an FDA analysis. On February 27, 2021, the FDA issued an EUA for Johnson & Johnson's Janssen vaccine, allowing it to be used to vaccinate adults in the United States. As of November 2023, approximately 19 million doses of the Janssen vaccine had been administered globally.

Controversy

State laws in the United States mandate that children in day care and students be immunized against certain diseases. Some exceptions are allowed. Still, many parents refuse to immunize their children for fear of a link between autism, for example, and the use of vaccines containing thimerosal, a mercury-based preservative. Although scientific evidence does not support this link, and thimerosal is no longer used in the production of most vaccines in the United States. To alert persons to adverse effects associated with vaccine administration, and to educate parents and others about what to expect after receiving a vaccine, an information sheet must be given to each person before he or she can be vaccinated.

Impact

Disease prevention is the key to public health, and it is always better to prevent a disease than to have to treat it. Vaccination is considered one of the most important medical discoveries in all of human history. Diseases can cause suffering, permanent disability, and death. Vaccines prevent disease in those who get vaccinated and protect those who come into contact with unvaccinated persons. Vaccination has controlled many infectious diseases that were once common, including polio, measles, diphtheria, pertussis (whooping cough), rubella (German measles), mumps, tetanus, and influenza. It even led to the complete eradication of smallpox from the human population.

Not all countries have the same level of vaccination requirements as the United States. Given the global nature of travel and business in the twenty-first century, exposure to many diseases is likely. Vaccination minimizes the risk of developing a disease and its associated complications. When persons travel outside the United States, additional vaccinations may be needed. One should consult a physician within a minimum of four weeks of traveling to determine what vaccines, if any, are needed.

Bibliography

Centers for Disease Control and Prevention. “General Recommendations on Immunization: Recommendations of the Advisory Committee on Immunization Practices.” Morbidity and Mortality Weekly Report, vol. 55, no. 47, 2006, pp. 1-48. www.cdc.gov/mmwr/PDF/wk/mm5547.pdf. Accessed 6 Mar. 2023.

“Immunization Schedules.” Centers for Disease Control and Prevention. www.cdc.gov/vaccines/schedules/index.html. Accessed 6 Mar. 2023.

Centers for Disease Control and Prevention. “Understanding the Basics: General Recommendations on Immunization.” Feb. 2023. www2.cdc.gov/nip/isd/ycts/mod1/courses/gbp/index.html. Accessed 9 Mar. 2023.

Merino, Noël. Vaccines. Greenhaven Press, 2015.

Plotkin Stanley A et al. Vaccines. 6th ed., Elsevier Saunders, 2013.

Shoenfeld, Yehuda, and Nancy Agmon-Levin. Vaccines and Autoimmunity. John Wiley & Sons, 2015.

"Types of Vaccines." National Institute of Allergy and Infectious Disease, 1 July 2019, www.niaid.nih.gov/research/vaccine-types. Accessed 7 Mar. 2023.

"How Vaccines Work." Centers for Disease Control and Prevention, 3 Feb. 2023, www.cdc.gov/coronavirus/2019-ncov/vaccines/different-vaccines/mRNA.html. Accessed 7 Mar. 2023.