A retroviral vaccine was a type of vaccine. Combined with biomimetic gels, these vaccines could create a biogenic weapon. In 2370, according to Ro Laren, a Pakled transport carried these vaccines a week after a Ferengi transport carried the gel, causing Starfleet to suspect the Cardassians were attempting to create such a weapon. In reality, this was a Starfleet trap for the Maquis. (TNG: "Preemptive Strike" )