Are Modified Live Vaccines Safe?

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302 – July/August 2019

by Megan Ulrich

It’s a world with its own vocabulary, an array of often confusing recommendations, and just as many ill-informed rumors. One commonly held misconception surrounds the use of modified live vaccines and their potential side effects in comparison to inactivated or killed vaccines.

According to the American Association of Equine Practitioners (AAEP), “Inactivated, or killed vaccines, lack pathogenicity and can neither replicate nor spread between hosts. These vaccines typically require multiple doses in the primary vaccinal series and regular boosters.” The AAEP defines modified live vaccines as those “typically derived from the naturally occurring pathogen, and are produced by: 1) attenuation in cell culture, 2) use of variants from other species, and 3) development of temperature-sensitive mutants.”

Confused yet? Dr. John Bengfort, DVM of Bluff Country Equine in Winona, Minnesota, defines the two in layman’s terms saying, “A modified live vaccine is one that most closely resembles the virus or bacteria it is intended to protect against. It has been modified, or attenuated, so that it doesn’t cause the disease, but because it most closely resembles the disease, it stimulates a stronger immunity. Think of a fly with its legs and wings removed. It’s still a fly but can’t act like a normal fly.”

He explains the difference from a killed vaccine, saying, “Killed vaccines are made by using viruses or bacteria that have been killed or torn apart. The appropriate protein particle—or particles—is then mixed with adjuvants that help increase the immune response. The fly comparison is that the fly is broken apart and those parts are used to make the vaccine.”

Click here to read the complete article
302 – July/August 2019