The virus has already affected the service’s operations. At least two members of the Secret Service who were in Tulsa, Okla., to help prepare for Mr. Trump’s campaign rally in June tested positive for the virus. The next month, Vice President Mike Pence changed his travel plans in Arizona after multiple agents tested positive or showed symptoms of the virus.
Rick Nelson, a former official on the National Security Council under the George W. Bush administration who dealt with homeland security issues, said few other law enforcement agencies faced such pressure to continue operations while at the whims of political candidates prone to travel.
“They have a relatively small work force that doesn’t allow them to absorb and respond to the fluctuation,” said Mr. Nelson, a fellow at the Center for Strategic and International Studies. “They’re at higher risk than the general public because they can’t do their job if they’re social distancing.”
The Secret Service trains its officers and agents at a sprawling campus in Laurel, Md. The center has mock versions of Air Force One, Marine One, the president’s helicopter and the White House grounds. There is a large parking lot where agents learn how to drive the hulking armored limousine the president is driven in, which is known as the Beast. A nearby garage houses many of the older limousines that the agency has used in recent decades.
As cities began to carry out social distancing guidelines in the spring, the Secret Service closed its training facility in Maryland from March to June 1. Agency officials said it reopened after four months in part to respond to mounting pressure to add to the nearly 7,800 employees in the agency. An inspector general report in 2016 found the agency’s training “continues to be hindered by low staffing levels and high operational demands on the work force.”
The agency enacted various safety procedures, including relocating classes outdoors, instituting temperature checks and requiring agents to wear personal protective gear while participating in close-contact drills, according to Julia McMurray, a spokeswoman for the Secret Service, who declined to provide a total number of confirmed cases among agents.
“Any U.S. Secret Service employee who may have tested positive would have been immediately isolated and returned home and out of the working environment. Considerations would also be taken to ensure the least amount of contact with the public,” Ms. McMurray said. “Throughout the pandemic, the U.S. Secret Service has taken significant precautions at its training center to protect the health and welfare of its trainees and training staff.”