A medical worker talks with a traveler returning from Croatia at a COVID-19 test center in Vienna, Austria, on Aug. 17, 2020. The city of Vienna built the drive-in and walk-in test center on the forecourt of the Ernst Happel Stadium, where returning Croatia travelers can be tested for a coronavirus infection free of charge. The offer valid until Aug. 21 applies to all returnees between Aug. 7 and Aug. 16. (Photo by Georges Schneider/Xinhua)

Medical workers are seen at a COVID-19 test center for travelers returning from Croatia in Vienna, Austria, on Aug. 17, 2020. The city of Vienna built the drive-in and walk-in test center on the forecourt of the Ernst Happel Stadium, where returning Croatia travelers can be tested for a coronavirus infection free of charge. The offer valid until Aug. 21 applies to all returnees between Aug. 7 and Aug. 16. (Photo by Georges Schneider/Xinhua)

A traveler returning from Croatia has a COVID-19 test at a test center in Vienna, Austria, on Aug. 17, 2020. The city of Vienna built the drive-in and walk-in test center on the forecourt of the Ernst Happel Stadium, where returning Croatia travelers can be tested for a coronavirus infection free of charge. The offer valid until Aug. 21 applies to all returnees between Aug. 7 and Aug. 16. (Photo by Georges Schneider/Xinhua)

Travelers returning from Croatia wait in their vehicles to have COVID-19 tests at a test center in Vienna, Austria, on Aug. 17, 2020. The city of Vienna built the drive-in and walk-in test center on the forecourt of the Ernst Happel Stadium, where returning Croatia travelers can be tested for a coronavirus infection free of charge. The offer valid until Aug. 21 applies to all returnees between Aug. 7 and Aug. 16. (Photo by Georges Schneider/Xinhua)