The question is a common one. Modified-live versus killed vaccines - which is best?

The bottom line is that the ultimate determination of a vaccine's merits comes from controlled tests conducted under field conditions similar to those of your production setting. But, these studies are difficult to conduct and have been conducted for only a few vaccines. Evaluating a vaccine's effectiveness is very difficult otherwise because so many management factors can overwhelm a vaccine's effect.

Viral or bacterial vaccines may be killed or live. Live vaccines contain bacteria or a virus that has been modified (MLV). This means they've lost their disease-causing ability (attenuated) or are administered by a route that prevents them from causing clinical disease. Killed vaccines are just what the name says - they've been attenuated though a process resulting in their death.

Advantages to both

There are advantages associated with both MLV and killed vaccines. The advantages of one are usually the disadvantages of the other.

Some of the positive attributes ascribed to MLV vaccines include:

  • A strong, long-lasting immune response that is achieved with fewer doses;
  • Adjuvants (immune stimulators) are not as necessary;
  • Virus vaccines may quickly stimulate non-specific, antiviral protection via interferon production;
  • The quality of the immune response that is stimulated can be different in ways that are currently thought to provide better protection. The details of these differences, however, are too complex to be presented here;
  • Less chance of allergic reactions; and,
  • The bacteria or virus may look and behave more like the disease-causing form of the organism.

Meanwhile, some of the advantages of killed vaccines are:

  • More stable on storage;
  • Unlikely to contain contaminating pathogens; and,
  • Unlikely to cause disease due to residual disease-causing characteristics.