The phrase "major vector" refers to a main carrier or transmitter of something, usually a disease or problem.
Sentences with «major vector»
It reduces feeding in fleas and stops the five other major vectors, quickly after application. (vectrapet.com)
Facebook grew in part by leveraging the work of established publications, then became a major vector for manipulative «fake news.» (fortune.com)
There are times when precision counts: Is that mosquito Anopheles dirus, which is a major vector of malaria, or A. harrisoni, which is not? (discovermagazine.com)