The phrase "primary vector" refers to the main or most important carrier or transporter of something. It is the primary way or method through which something is transmitted or spread. Full definition
In a recent test of Asian tiger mosquitoes collected in Brazil, researchers found fragments of Zika virus RNA, raising concerns that it may be carried by species other than Zika's known primary vector, the yellow fever mosquito. (sciencedaily.com)
«Traces of Zika Found in Asian tiger mosquito in Brazil: Virus fragments detected in species other than Zika's known primary vector.» (sciencedaily.com)
If you are unwilling to admit that acts of sodomy are the primary vector (Disease transmission path) for the AIDS virus, then there is no point in continuing the conversation. (religion.blogs.cnn.com)