There IS a wide variation in patients’ responses to acupuncture and electroacupuncture (EA.)  In this study they look at gene expression that might explain those differences.  I’ve seen in my own studies that vagal enhancement is more often seen during treatment in responders, and the enjancement can be dramatic in “profound responders.”

PLoS One. 2012;7(8):e42331. doi: 10.1371/journal.pone.0042331. Epub 2012 Aug 3.

Differences in neural-immune gene expression response in rat spinal dorsal horn correlates with variations in electroacupuncture analgesia. Wang K, Zhang R, Xiang X, He F, Lin L, Ping X, Yu L, Han J, Zhao G, Zhang Q, Cui C. Source

Shanghai-MOST Key Laboratory of Health and Disease Genomics, Chinese National Human Genome Center at Shanghai and National Engineering Research Center for Biochip at Shanghai, Shanghai, China.
Abstract
BACKGROUND:

Electroacupuncture (EA) has been widely used to alleviate diverse pains. Accumulated clinical experiences and experimental observations indicated that significant differences exist in sensitivity to EA analgesia for individuals of patients and model animals. However, the molecular mechanism accounting for this difference remains obscure.
METHODOLOGY/PRINCIPAL FINDINGS:

We classified model male rats into high-responder (HR; TFL changes >150) and non-responder (NR; TFL changes = 0) groups based on changes of their pain threshold detected by tail-flick latency (TFL) before and after 2 Hz or 100 Hz EA treatment. Gene expression analysis of spinal dorsal horn (DH) revealed divergent expression in HR and NR after 2 Hz/100 Hz EA. The expression of the neurotransmitter system related genes was significantly highly regulated in the HR animals while the proinflammation cytokines related genes were up-regulated more significantly in NR than that in HR after 2 Hz and 100 Hz EA stimulation, especially in the case of 2 Hz stimulation.
CONCLUSIONS/SIGNIFICANCE:

Our results suggested that differential regulation and coordination of neural-immune related genes might play an important role for individual variations in analgesic effects responding to EA in DH. It also provided new candidate genes related to EA responsiveness for future investigation.