Trial aims to study the safety and immunogenicity of the vaccine in healthy adults aged 18 to 64 years or 65 and older.
Pregnant women, young children, and the elderly are especially susceptible to severe Listeria infections.
Infections linked to packaged salads from 2 different firms caused 30 illnesses.
Seven new illnesses and 2 additional deaths have been confirmed in the outbreak, which has grown to 27 cases and 6 deaths.
Twenty people in 15 states have been now sickened in the outbreak, and 4 have died.
The CDC's FoodNet program has reduced required surveillance from 8 foodborne pathogens to 2, a move some experts warn could leave the country more vulnerable to outbreaks.
So far 40 states have reported measles cases, with 88% of infections part of 29 outbreaks.
A pregnant woman who was sickened has also reported fetal loss following her infection.
An earlier investigation hinted at hospital food as a source, but evidence didn't reveal a specific item.
FDA and USDA teams are still working to identify a food source for the Listeria outbreak.
