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Comparison of Vaccine Formulations: Modified Live, Killed
& Toxoid Vaccines
Difference Between the Inactivated and Attenuated Vaccinations
By Tami Port, MS
With the current double-barrel influenza threat of seasonal and H1N1 swine flu, the safety of vaccines is on everyone's
mind. Should you get a live or killed virus vaccine? What is the difference, and what are the pros and cons of each?
The following is a comparison summary of the different categories of vaccine available.
Attenuated, Live Vaccines
Vaccines that are considered attenuated contain modified live viruses (MLV) or other disease-causing microbes, that
have been altered in a way that weakens them to the point that they are no longer virulent, and are unable to cause
disease. Viruses are typically made avirulent, by using a virulent stain to infect laboratory tissue cultures and then
having the viruses reproduce for many generations until they lose their ability to cause disease.
The pathogenic microbes in attenuated vaccines, although avirulent, must still be active, and able to replicate after
inoculation so that they achieve adequate numbers to stimulate the body's immune response.
Pros and Cons of Attenuated Live Vaccines: * Advantage: Usually only one dose required.
* Advantage: Stimulates rapid immune response after inoculation.
* Advantage: The body's immune response to the pathogen is stronger and lasts longer.
* Disadvantage: Although weakened, attenuated microbes may still have enough virulence to cause disease in
immune compromised patients.
* Disadvantage: Rarely, attenuated microbes can revert to wild type, that are able to cause disease.
Killed Inactivated Vaccines
Inactivated vaccines contain microbes that have been treated by chemical or physical means to prevent them from
replicating, or causing disease. Some inactivated vaccines contain the entire infectious agent (whole agent vaccines),
while others include only antigenic parts of the microbe (subunit vaccines) that still stimulate the the body to launch
and immune response.
Pro and Cons of Inactivated Vaccines: * Advantage: Safer because there is no risk of the infectious agent replicating
or reverting to a virulent form.
* Advantage: Recommended for immune compromised and pregnant patients.
* Disadvantage: Antigenically weak; several booster vaccines may be needed to achieve complete immunity.
* Disadvantage: In whole agent vaccines, the nonantigenic portions of the microbe may result in a painful inflammatory
response in some individuals.
Toxoid Vaccines
This type of vaccine is used to inoculate against some bacterial diseases, in which the bacterium produces a toxin.
These vaccines contain a modified toxin produced by the microbe in question, which initiates an immune response in
the person vaccinated. Like subunit inactivated vaccines, toxoid vaccines are generally safer than attenuated live
vaccines, but induce a weaker immune response, so require multiple doses.
Difference Between the H1N1 Flu Mist and Injection
This flu season, there will be two types of swine flu vaccine available. The flu injectable vaccine is made with
inactivated, "killed" H1N1 virus. In contrast, the mist contains an attenuated or weakened virus. The CDC indicates
that the safety and efficacy data for both formulations of vaccine--nasal spray and injection--are very comparable
To learn more about the official recommendations regarding the general immunization schedule and seasonal and
swine flu vaccination, see the information on the CDC's influenza web pages, or read the Suite101 article on simple
steps to help avoid becoming sick with influenza.
Sources
Bauman, R. (2004) Microbiology, Pearson Benjamin Cummings.
Center for Disease Control (CDC), H1N1 Flu Web Pages, updated as of October 9, 2009.
National Public Radio, Diane Rehm Show, H1N1 Vaccine Plans, aired on October 7, 2009.
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Please note: The information in this article is not intended as medical advice. Please consult your physician provider
regarding your own personal health and necessary treatments. This article was originally published in Suite101 online
magazine.
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