WASHINGTON – The Agriculture Research Service (ARS) of the US Department of Agriculture announced that a vaccine candidate for African swine fever (ASF) does not revert to normal virulence after being injected into pigs. This safety test is required for regulatory approval because it shows that the vaccine’s weakened form of ASF doesn’t revert to its original state.

“An attenuated vaccine strain retains much of its genetic makeup and could genetically change when exposed to various external circumstances,” ARS said. “All live attenuated vaccines are weakened versions of a virus and can be used as a vaccine because the live vaccine virus will not cause illness and can still provide immunity.”

Passing this important safety test moves the vaccine closer to commercial availability, ARS said. The research is featured in the April issue of “Viruses.”

“It is very hard to predict how selective pressure can cause a live attenuated vaccine to return to virulence,” said Douglas Gladue, senior scientist at ARS. “In the case of this particular vaccine candidate, ASFV-G-ΔI177L, we deleted a gene, which makes it difficult for the virus to simply add the gene back. So, we expected reversion to its original form to be unlikely, but the test still has to be performed.”

The vaccine candidate was recently selected by the National Veterinary Joint Stock Company (NAVETCO) for commercial development in Vietnam, ARS said. NAVETCO is owned by the Ministry of Agriculture and Rural Development. Safety studies are necessary to gain approval for use in Vietnam and eventually in other countries. Future commercial use, however, will depend on approval from the department of animal health within each requesting country, according to ARS.

“This is a critical milestone for the ASF vaccine candidate. These safety studies bring this vaccine one step closer to being available on the market,” said Manuel Borca, a senior scientist with ARS.