Yellowstone National Park has continued to operate without a Covid-19 outbreak/NPS file

Editor's note: This updates with the remaining 121 tests coming back negative.

Since Yellowstone National Park reopened in mid-May, nearly 1,500 park and concession workers have been tested for Covid-19 as part of an asymptomatic surveillance program without a positive result returned, according to park staff. 

Earlier Wednesday the number was 1,378, but during the day another 121 outstanding tests came back negative, staff said, bringing the total to 1,499.

There were two concession workers who tested positive for Covid-19 in July, and both have recovered, a park release said. 

"The first employee experienced COVID-19 symptoms while away from the park and was tested outside the park. Once the positive test was confirmed, the employee was isolated," a park release explained. "Based on contact tracing by health officials, the employee had limited interactions with visitors or employees. It is likely this employee contracted the virus while out of the park.

"The second employee had symptoms while in the park, was tested at the clinic, and determined to be positive. The employee was immediately isolated while contact tracing occurred."

No visitors have tested positive in the last four weeks in any of Yellowstone’s medical clinics, according to park staff. There have been approximately 1.66 million visits to the park since it opened on May 18.