Risk of chikungunya virus transmission associated with European travelers returning from southern Thailand (2008-2015)

BACKGROUND The impact of the spread of chikungunya virus (CHIKV) by autochthonous transmission and blood transfusion in nonendemic areas via travelers returning from CHIKV-affected locations is a concern. METHODS We analyzed the risks of potential CHIKV importation and transfusion transmission from Thailand to Europe via travelers visiting southern Thailand from 2008 through 2015, using the web-based European Up-front Risk Assessment Tool. RESULTS The risk of CHIKV importation by European travelers returning from Thailand from 2008 through 2015 varied depending on the year of travel, tourist destination, duration of stay, and time since last possible exposure. Specifically, the risks of acquiring CHIKV among travelers visiting Songkhla and Krabi for 1, 5, or 10-30 days during the highest epidemic activity in 2009 were estimated to be 74.40, 371.99, and 706.77 (Songkhla) and 1.82, 9.08, and 17.25 (Krabi) per 100,000 travelers, respectively. In contrast, such risks were estimated to be fewer than 0.099 per 100,000 travelers in nonepidemic years. The 2009 yearly average rates of expected incidence among 4,059,988 European travelers who stayed for 1 or 10-30 days in all six outbreak activity destinations were calculated to be, respectively, 4.01 x 10(-6) or 1.20 x 10(-4) cases per day, corresponding to the estimated rates of viremia and transfusion-transmitted CHIKV via traveling blood donations of 3.21 x 10(-5) and 0.61, and 9.62 x 10(-4) and 3.34, respectively. Additionally, it is probable that 18 (0.0004%) Europeans acquired CHIKV in Thailand, representing a maximum attack rate of 0.0023%. CONCLUSION The extent of the expected risks and attack rates of CHIKV infection might reflect the travel preferences for popular destinations rather than the true risks of CHIKV transmission in travelers' home nonendemic countries. Nevertheless, preventive and blood-safety intervention measures may be applied to returning travelers at risk for infection to reduce CHIKV transfusion threats in their home countries.