The virus outbreak could hit U.S. plans to return to the Moon by 2024.
"We realize there will be impacts to NASA missions, but as our teams work to analyze the full picture and reduce risks we understand that our top priority is the health and safety of the NASA workforce", Bridenstine said.
While Bridenstine noted there would be "impacts to NASA missions" due to the closures, including to work on the Orion spacecraft - created to bring American astronauts to the Moon by 2024 - he said the health of personnel remained a "top priority".
NASA and SpaceX are targeting mid- to late May for the launch of Demo-2, which will send agency astronauts Doug Hurley and Bob Behnken to the International Space Station (ISS) aboard SpaceX's Crew Dragon capsule, NASA officials confirmed in a media advisory on Wednesday (March 18).
Work on the SLS, led by Boeing Co as the prime contractor, has been dogged by years of delays and almost $2 billion in cost overruns.
The agency's Ames Research Center in California has also been shuttered under a Stage 4 order after another employee tested positive for Covid-19 last week, while a number of other facilities - including Houston's Johnson Space Center - have been moved to Stage 3, limiting the number of workers and other staff allowed on site amid the viral outbreak.
Both the Stennis Space Center in southern MS and the Michoud Assembly Facility about 40 miles away in New Orleans will halt operations, NASA Administrator Jim Bridenstine said in a statement on Thursday.
The US Apollo program, NASA's forerunner to the current lunar effort, accomplished the world's first six and only manned missions to the moon between 1969 and 1972.