Epidemiology, News

How Vaccines Work

Author Jaclynn Moskow, 07-05-2021

Table of contents

Development and manufacture process of a new vaccine. Final production of filled vials of Covid-19 vaccine. Bio science 3D illustration.

 

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. 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

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.

 

How inactivated vaccines work

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 does 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”.

 

How vaccines work: inactivated vaccine

Inactivated vaccine mechanism of action

 

How subunit vaccines work

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.

 

Messenger RNA (mRNA) – a new type of vaccine

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. 

 

How vaccines work: mRNA vaccine schematic illustration

mRNA vaccine mechanism of action

 

How viral vector vaccines work

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.

 

How vaccines work: viral vector vaccine

Viral vector vaccine mechanism of action

 

 

Dr. Steve Berger on Vaccines

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.”

 

Optimizing Immunity

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. This is discussed in more detail in our blog: Strengthen Your Immune System! Your Guide to the Ultimate 2021 New Years Resolution

 

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References:

(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]. Available: https://www.cdc.gov/mmwr/preview/mmwrhtml/00046568.htm.

Author
Jaclynn Moskow

Jaclynn M Moskow D.O. is a professional medical writer and freelance healthcare consultant. Dr. Moskow has an extensive research background, having conducted and published bench research, clinical research, and translational research. She attended the University of Pittsburgh Honors College, where she designed and earned a Bachelor of Philosophy in Molecular Biology, Chemistry, and the History of Medicine. She earned her Doctor of Osteopathic Medicine from Nova Southeastern University, where she went on to serve as a Clinical Instructor of Public Health.

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