So even the conservative CATO institute is like:

“one terrorism-planning conviction for a refugee for every 286,543 of them who have been admitted. To put that in perspective, about 1 in every 22,541 Americans committed murder in 2014. The terrorist threat from Syrian refugees in the United States is hyperbolically over-exaggerated and we have very little to fear from them because the refugee vetting system is so thorough.”
Syrian Refugees Don’t Pose a Serious Security Threat

The current refugee vetting system is multilayered, dynamic, and extremely effective.

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