HOW VACCINES WORK
A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains components (antigens) of a disease agent or inactivated forms of the entire agent which may be bacteria, viruses or toxins. A vaccine stimulates the body's immune system to recognize the antigen as foreign, and then to destroy and "remember" it. Upon renewed encounter of the disease agent, immune memory then enables its rapid destruction before disease symptoms develop.
COMMON TYPES OF VACCINES
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PROTEIN SUBUNIT VACCINES
This type of vaccine is composed of specific antigenic proteins of pathogens and is made by recombinant DNA technology that avoids ever handling the actual whole live or inactivated disease agent. Protein subunit vaccines can elicit protective immune memory and specific neutralizing antibodies and have a favorable safety profile that permits administration to vulnerable patients such as those that are immunocompromised. HPV vand Hep B vaccines are established recombinant protein subunit vaccines. A new recent example is MVC's COVID-19 vaccine that is composed of the highly antigenic SARS-CoV-2 S (spike) surface protein.
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INACTIVATED VACCINES
Chemicals, heat or radiation are used to inactive disease agents before they are used as vaccines. Proper inactivation ensures that these vaccines are safe and they are generally easy to transport and store. Flu and Hep A vaccines are examples.
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ATTENUATED LIVE VACCINES
Attenuated live vaccines are made up of pathogens that have been sufficiently weakened so that they are nonpathogenic. These vaccines may not be suitable for immunosuppressed people, but are the only option for creating vaccines with disease agents where an immunogenic vaccine cannot be generated by another vaccine production platform. Rotavirus and MMR vaccines are examples.
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NUCLEIC ACID VACCINES (mRNA/DNA)
The nucleic acid approach is a new way of developing vaccines where the mRNA/DNA encodes an antigenic protein of the disease agent. Upon injection, the host cells "read" the mRNA/DNA and produce the protein which then stimulates the immune system to generate neutralizing antibodies and establish immune memory. Nucleic vaccines can be deployed quickly because time-consuming cell culture technology is not required to produce the protein antigen as an injected vaccine. To date, the COVID-19 mRNA vaccines are the only examples of such vaccines.
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VIRAL-VECTORED VACCINES
A nonpathogenic virus is equipped with the genetic information (DNA) that encodes an antigen from the disease agent. Similar to nucleic acid vaccines, injection of this virus delivers the DNA to human cells which then "read" the DNA to produce antigen that induces immune responses. Adenovirus, measles, and vesicular stomatitis virus (VSV) are commonly used as vectors. Two viral-vectored vaccines, Ebola vaccines and COVID-19 vaccines, have been used in humans.
Reference:
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Callaway E. The race for coronavirus vaccines: a graphical guide. Nature. 2020 Apr;580(7805):576-577. https://doi.org/10.1038/d41586-020-01221-y
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Kyriakidis, N.C et al. SARS-CoV-2 vaccines strategies: a comprehensive review of phase 3 candidates. npj Vaccines 6, 28 (2021). https://doi.org/10.1038/s41541-021-00292-w
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WHO (2021). The different types of COVID-19 vaccines. World Health Organization. https://www.who.int/news-room/feature-stories/detail/the-race-for-a-covid-19-vaccine-explained
Value of Vaccines
- Vaccines are critical for efficient establishment of herd immunity.
- Commercially-available vaccines exist for over 30 diseases and they save over 3 million lives every year.
- According to US CDC, for every 1 dollar spent on vaccination, 16 dollars will be saved by avoiding disease treatment and productivity losses.
