MRSA infections in Norway: A study of the temporal evolution, 2006-2015

Background Norway has one of the lowest prevalences of methicillin-resistant Staphylococcus aureus (MRSA) infections in the world. This study exploits the extensive data on MRSA infections in the Norwegian surveillance system to investigate the important factors defining the MRSA epidemiology. Methods We performed a quasi-Poisson regression of the monthly notification rate (NR) of MRSA infections reported from January 2006 to December 2015, comparing the time trend among people with an immigrant vs. Norwegian background and domestic vs. imported infections, stratified by age groups. Findings A total of 5289 MRSA infections were reported during the study period, of which 2255 (42.6%) were acquired in Norway, 1370 (25.9%) abroad, and 1664 (31.5%) with an unknown place of acquisition. Overall, the monthly NR increased significantly from 2006 to 2015 (+0.8% each month). The monthly increase in immigrants (+1.3%) was steeper than that in people with a Norwegian background (+0.6%). There was a significant growth (+0.4%) in the rate of domestically acquired infections, however, the NR of infections acquired abroad increased faster (+0.8%). For both imported and domestic infections, the increase occurred in persons aged < 70 years. Interpretation Our analysis suggests that immigration and importation, especially among persons aged < 40 years, represent important factors for the increasing notification rate of MRSA infections in Norway.