Background: Comprehensive background check (CBC) laws extend background check requirements to private party firearm transfers to prevent firearm acquisitions by prohibited persons. The aim of our study was to estimate the association between CBC policies and changes in background check rates for firearm acquisition in two states (Oregon and Washington) that have newly-enacted CBC policies. Methods: We used data on handgun background checks from January 1999 to December 2018 from the National Instant Criminal Background Check System. Observed trends in exposed states were contrasted with counterfactual trends estimated with the synthetic control group method. Findings: CBC policies were associated with increases in background checks in Oregon (by 18.0%; p = 0.074), but not in Washington (4%; p = 0.321). A gradual increase in private party checks was seen following enactment in Washington; however, firearm transactions coded as "private" represent less than 5% of total background checks in that state. Conclusions: Comprehensive background check policies appear to be effective in increasing pre-firearm-sale background checks in Oregon but not in Washington. Differences appear to be related to variations in the proportion of firearm sales that are private party transfers and to gradual adaptation to the new law by private gun sellers.
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