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You may know they save lives, but have you ever wondered how vaccines work? As mass vaccination programs in response to the COVID-19 pandemic are underway, there is a heightened interest in understanding vaccines and their mechanism of action.
Vaccines stimulate the immune system to recognize and respond to specific pathogens. This is part of why people refer to being immune to disease after getting vaccinated. Differing vaccines accomplish this through a variety of mechanisms. Vaccines can be divided into five basic categories: live attenuated vaccines, inactivated vaccines, subunit vaccines including toxoids, mRNA vaccines, and viral vector vaccines.
Live attenuated vaccines contain live viruses or bacteria that have been weakened to reduce virulence. Following ingestion or injection, the body mounts an immune response that is similar to what would occur if it were to encounter the natural disease. Since the pathogens used in live attenuated vaccines have been weakened, they cause only mild symptoms – or no symptoms at all. Live attenuated vaccines generally provide the longest-lasting immunity of any vaccine type.
Examples of live attenuated viral preparations include measles, mumps, rubella, varicella, rotavirus, yellow fever, oral polio vaccine, and intranasal influenza vaccine. Live attenuated bacterial vaccines include the BCG vaccine for tuberculosis and the oral cholera vaccine. In general, viral vaccines have greater efficacy than bacterial vaccines.
The eradication of smallpox was accomplished through a live attenuated vaccine, which contained a related virus (vaccinia). In the veterinary world, widespread use of a live attenuated vaccine led to the eradication of Rinderpest, also known as ‘cattle plague’, caused by a morbillivirus of the family Paramyxoviridae.
Live attenuated vaccines are generally contraindicated in pregnant women and immunocompromised patients, such as those receiving immunosuppressive therapy or living with HIV/AIDS or congenital immunodeficiency.
It is extremely rare for a live attenuated vaccine virus to mutate into a more virulent form and cause disease. Cases of this phenomenon have been documented with oral polio vaccine, at a reported rate of one case per 750,000 children receiving their first dose (1). As a result of these rare instances, in many countries (including the United States), the oral polio vaccine is no longer used and has been replaced by inactivated preparations.
Inactivated vaccines, also known as killed vaccines, prime the immune system with bacteria or viruses that have been inactivated to remove all virulence. They cannot cause the disease they protect against and are generally considered safe for immunocompromised and pregnant patients. The protection provided by inactivated vaccines do not usually last as long as that provided by live attenuated vaccines, and booster doses are often recommended.
Examples of inactivated vaccines include hepatitis A, rabies, intramuscular influenza, intramuscular polio, and variants of the pertussis vaccine. Several inactivated SARS-CoV-2 vaccines have been developed, including China’s “Sinovac”, India’s “Covaxin”, and Russia’s “CoviVac”.
Subunit vaccines contain fragments of a pathogen (i.e., a polypeptide or polysaccharide) often bound to other molecules. As with inactivated vaccines, these cannot produce the disease itself and are generally considered safe for immunocompromised and pregnant patients. Booster doses are often required.
Examples of subunit vaccines include hepatitis B, human papillomavirus (HPV), Haemophilus influenzae type B (HiB), herpes zoster, meningococcus B, pneumococcal, and one variant of the pertussis vaccine. Occasionally, the subunit used in these vaccines is an attenuated toxin (toxoid). Examples of toxoid vaccines include tetanus and diphtheria.
There are several subunit vaccines for SARS-CoV-2 in various stages of clinical trials around the world.
mRNA vaccines are a new type of vaccine and contain fragments of mRNA that encode a piece of protein from the pathogen of interest. After being vaccinated with such a vaccine, the body’s own cells incorporate the mRNA and produce the protein, which the immune system then recognizes as foreign. The Pfizer-BioNTech and Moderna COVID-19 vaccines are both mRNA vaccines. There are several additional mRNA vaccines currently in development.
Viral vector vaccines use modified versions of viruses as “vectors” to deliver a nucleic acid of the pathogen of interest into the cell. Once inside the cell, the DNA is transcribed into mRNA, and the mRNA is translated into protein. The body then recognizes this protein as foreign and mounts an immune response, similar to that which occurs with mRNA vaccines. The Johnson & Johnson COVID-19 vaccine works in this manner.
GIDEON co-founder Dr. Steve Berger reflects on vaccines: “Vaccines continue to save millions of lives and have prevented untold misery to the human species. Although the effectiveness of individual vaccines may vary, and most may cause occasional side effects, the cost of non-vaccination – in both death and suffering – will always be much higher.”
Proper nutrition, exercise, ample sleep, and adequate levels of Vitamin D have been shown to enhance the efficacy of vaccines as well as strengthen the immune system as a whole. So aside from being vaccinated, there are also things that you can do to keep your health a priority. Information on the immune system, building immunity, and more is critical for keeping your body optimized for combatting disease whether from bacteria or a virus. This is discussed in more detail in our blog: Strengthen Your Immune System.
GIDEON is one of the most well-known and comprehensive global databases for infectious diseases. Data is refreshed daily, and the GIDEON API allows medical professionals and researchers access to a continuous stream of data. Whether your research involves quantifying data, learning about specific microbes, or testing out differential diagnosis tools– GIDEON has you covered with a program that has met standards for accessibility excellence.
(1) “Poliomyelitis Prevention in the United States: Introduction of A Sequential Vaccination Schedule of Inactivated PoliovirusVaccine Followed by Oral Poliovirus Vaccine; Recommendations of the Advisory Committee on Immunization Practices (ACIP)“, Cdc.gov, 1997. [Online]