Live attenuated vaccines are made by weakening the natural virus or bacteria (uncommon).
Viruses are so very simple, in that they contain very few genes. It’s because of this that researchers are very able to control those genes. Viruses are often attenuated (weakened) by growing them in cells that they don’t normally grow in for many generations. The viruses begin to evolve and adapt to these new cells so that they are less able to live in their preferred environment. Therefore, when attenuated viruses enter the human host, they are not able to reproduce and create disease like they would naturally.
Natural viruses need to reproduce thousands of times during infection to cause disease. These weakened viruses are only able to reproduce fewer than twenty times.1 With such little reproduction, attenuated viruses are not able to create nearly enough copies of themselves to cause disease. However, enough viral particles are created to create memory antibodies and keep the body from getting the viral infection should it enter the body naturally in the future.
Advantages
- Live attenuated vaccines are great at teaching the immune system how to fight off a particular virus because they are the closest to a natural infection.
- They only require one to two doses for life-long immunity. (Fewer boosters!)
- These vaccines are relatively easy to create for certain viruses.
Disadvantages
- It is possible that weakened viruses have the ability to gain control of their natural environment again and cause disease.
- These vaccines can not be administered to people with weakened immune systems due to cancer, HIV, or other immune system depressing disease.
- Live, attenuated vaccines usually must be refrigerated and protected from light. It may be hard to ship these vaccines overseas and used in places that lack refrigeration.
- This technique doesn’t work as well with bacteria, therefore there are very few live bacterial vaccines. Viruses are very simple and contain very few genes in relation to bacteria, which have thousands of genes. This makes bacteria much harder to control and manipulate than viruses. Currently, scientists are hard at work trying to remove key genes from certain bacteria, in order to create weakened versions to be used in vaccines.3
The vaccinations that use this strategy are:
Viral:
- MMR Vaccine
- Rotavirus Vaccine
- Oral Polio Vaccine (not used in the USA)
- Some Influenza Vaccines
- Varicella (chickenpox) Vaccine
- Shingles Vaccine
- Yellow Fever Vaccine
- Adenovirus Oral Vaccine (military)
- Vaccinia (Smallpox) Vaccine
Bacterial:
- BCG (tuberculosis) Vaccine (not used in the USA)
- Typhoid Oral Vaccine (not used in the USA)
- Live Cholera Vaccine (not used in the USA)
Resources:
- The Children’s Hospital of Philadelphia (CHOP). www.chop.edu
- The Centers for Disease Control and Prevention (CDC). www.cdc.gov
- National Institute of Allergy and Infectious Disease. National Institutes of Health. www.niaid.nih.gov
- The history of Vaccines. The College of Physicians of Philadelphia. www.historyofvaccines.org
- Vaccine Healthcare Centers Network. www.vhcinfo.org