The phrase "primary effect" basically refers to the main or most important consequence or outcome of something. It focuses on the main result or impact that is significant or influential. Full definition
Whatever rules of conduct the particular adult may specify, one primary effect for the child is to confirm the capacity that is already at work in him or her. (firstthings.com)
This is obvioulsy not the whole story but it is worth keeping in mind regarding the MOC whose primary effect is to redistribute heat. (realclimate.org)
In the case of the cancer drug L - asparaginase type 2 (L - ASN2), whose primary effect is depleting asparagine, side effects are generally attributed to the corresponding depletion of a chemically similar molecule called glutamine. (sciencedaily.com)