Salesforce and Twitter have barred most employees from flying for business within the United States — moves that could herald a wider impact on domestic air travel.

Salesforce, San Francisco’s largest private employer, announced Monday it is restricting “all but the most critical domestic travel,” in addition to other precautions, due to coronavirus concerns. Twitter made a similar move over the weekend, and Amazon has done so, too.

Companies had already banned much international travel, particularly to areas hard-hit by the virus, but domestic travel is now being jettisoned, too.

Ben Mutzabaugh, senior aviation editor at travel website the Points Guy, said companies like Twitter and Amazon have an “incredibly outsized effect on the airline business.”

“They’re really contributing a lot to the revenue that airlines take in,” he said. “Leisure travelers are impactful, but it’s these giant corporate accounts that really make the difference for airlines, both based on volume and expense of fares, they travel a lot and tend to travel last minute.”

“If they’re pulling out, airlines are feeling it very quickly,” he added. “Then the question is how long are they going to keep flying planes that are relatively empty? The answer is, we don’t know.”

Air travel demand has been hurt by a rash of cancellations of business conferences, which shows no signs of stopping. On Monday, Santa Clara chipmaker Nvidia announced that its GTC conference, scheduled for late this month in San Jose, would become a digital-only event.

Mutzabaugh said he was sure that in “every corporate headquarters of a U.S. airline are nervous executives wondering just how bad this is going to get.”

Local airports and the region’s major airlines did not report a change in domestic flight schedules Monday. San Francisco International Airport spokesman Doug Yakel said there were no domestic reductions.

But airport workers beyond the International Terminal say they’ve noticed a slowdown. AJ Cruz, a barista at Green Beans Coffee in SFO’s Terminal 1, said he’s noticed a lot of airport workers coming by but fewer passengers.

“A little quieter,” he said.

The shop is still open 24 hours. He said his employer told him he was welcome to wear a mask.

“Other than that, just to stay cautious and everything,” he said.

At SFO on Monday, travelers were taking precautions. Andrew Hoffman, a 37-year-old Bay Area resident who’s an executive with a tech company, covered his mouth and nose with a multicolored mask as he headed toward his flight to Colorado.

“I’m going to be as careful as I can be,” said Hoffman, who also carried antibacterial wipes. He travels once a month for meetings and said he could do them virtually, but it’s always better in person. He said his business is holding off on plans for a company gathering in April while monitoring the situation and taking precautions.

“We haven’t wholesale canceled everything so far,” he said. “I don’t think anyone has a crystal ball.”

Henry Harteveldt, a travel analyst with Atmosphere Research in San Francisco, noted that the virus situation poses a stark dilemma for businesses.

“Companies are very concerned about the impact of suspending business travel on their sales and their ability to support their customers,” he said. “At the same time, they recognize they have a responsibility to their employees’ health and well-being.”

Harteveldt said future travel impacts are hard to predict because the situation is changing so quickly, but since now is the prime season for booking summer vacations, he expected to see more delayed bookings closer to summer travel dates as potential passengers wait to see how things unfold.

As travelers change plans, some airlines are waiving rebooking fees. American Airlines, for example, said Sunday that it will waive normal change fees on any ticket purchased between March 1 and March 16 for travel until Jan. 26.

Alejandro Mayorkas, former deputy secretary of the U.S. Department of Homeland Security who is now a partner at the law firm WilmerHale, said companies weren’t necessarily liable from a legal perspective if they allow employees on flights.

Mutzabaugh said budget airlines that cater to leisure travelers will also see the impact as passengers “get skittish about traveling, families will stop traveling, and those airlines will be hit.” He said he didn’t expect to see any cancellations, just emptier flights, unless an outbreak worsened in a particular region in the U.S. and officials wanted to cut off access to try and contain it.

Southwest, which accounts for 70% of traffic at Oakland International Airport, has not suspended scheduled flights or routes due to the coronavirus and does not currently serve regions where the virus has been noted as particularly problematic.

“Southwest will continue to monitor and follow all guidance from the Centers for Disease Control regarding the coronavirus and make any adjustments to our operations, if necessary,” spokesman Brian Parrish said in an email Monday.

Mineta San Jose International Airport spokeswoman Demetria Machado said in an email Monday that other than a flight route to China that was already cut, there have been no significant changes like other canceled flights, route reductions or plane size changes. In fact, British Airways will be switching aircraft for its direct service to London to a 747 by the end of this month to allow for more seat capacity, she added.

Salesforce said in its blog post Monday that it was “replacing our in-person customer events with digital experiences, and enhancing our office protocols to ensure we provide the healthiest work environment possible.” It was not immediately clear whether the policy change would affect Dreamforce, the company’s 171,000-person conference in San Francisco in the fall.

Twitter went further Monday, urging its 5,000 workers to work from home, though it is not closing its San Francisco headquarters or other offices. In some countries, its employees must do so because of government restrictions imposed to slow the spread of the coronavirus.

Harteveldt said he was hearing from travel managers, airlines and hotels that the expectation that business travel restrictions could be limited in scope to a few weeks, but “every conversation ends with the statement, ‘but we’ll have to see what happens.’”

Low demand for air travel could bring down prices, he said.

“You never want to put yourself at risk of becoming ill just to take advantage of a good airfare or hotel price, but to destinations that they feel are safe to visit, like Hawaii, Florida and Mexico, there may be some great opportunities for last-minute getaways,” he added.

Chronicle columnist Kathleen Pender and staff writer Roland Li contributed to this report.

Chase DiFeliciantonio and Mallory Moench are San Francisco Chronicle staff writers. Email: chase.difeliciantonio@sfchronicle.com, mallory.moench@sfchronicle.com Twitter: @ChaseDiFelice, @mallorymoench