The vaccine is essentially a hybrid virus which has been made to grow in the cowpea plant, the source of black-eyed beans.
From the newscientist.com
Unfortunately, as the cowpea plant evolved over time, so did a parasitic plant that starves the cowpea plant to death by sucking nutrients from its roots.
From the sciencedaily.com
Farmers also plant cowpea among crops with leaves that provide too much shade and thus impede its growth.
From the voanews.com
The older cowpea varieties are also susceptible to a parasitic plant called striga, which competes with the food crop for moisture and nutrients.
From the voanews.com
Researchers also will discuss endowing cowpea varieties with a gene from a common bean plant to provide protection infestations during storage.