Live and Killed Vaccines

With last year’s H1N1 swine flu outbreak and this season’s whooping cough epidemics, vaccines, and their safety are 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 Flu Mist and Injection *

This flu season, there will be two types of flu vaccines available. The flu injectable vaccine is made with inactivated, “killed” influenza viruses. 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 general immunization schedules and seasonal influenza vaccination, see the information on the Center for Disease Control’s (CDC’s) official website.

* 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.

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 first published in Suite101 online magazine.