The Trump administration announced Sunday that some of the most vulnerable Americans will be able to get tested for the novel coronavirus from their cars starting this week — a significantly less ambitious program than the swift nationwide testing campaign President Trump promised Friday.

At a news conference, Vice President Pence and federal health officials said the first people allowed to use drive-through testing will be health-care workers and first responders, as well as people over 65 who have symptoms consistent with the virus, such as a cough.

The officials did not explain exactly where or in how many states the drive-through tests would begin, other than to say it would be in hard-hit areas. And they backed away from an announcement by Trump on Friday that Google was on the verge of releasing a website through which any American could type in symptoms and learn whether they warranted a test.

But the vice president and Brett Giroir, an assistant Health and Human Services secretary put in charge of coordinating coronavirus testing late last week, said the ability to get swabbed in store parking lots would expand over time, with the Federal Emergency Management Agency and the U.S. Public Health Service helping to equip states with test kits, training, protective gear and staff as testing widens across the country.

In playing down the president’s announcement two days earlier about a Google website, Pence said, “We are working with Google, but we are working with many other tech companies.” He added, “Our best estimate . . . is that some point early in the week we will have a website that goes up” to help worried Americans determine whether they should get tested, and then “more and more of these sites come online, working with state government and FEMA and local businesses and retailers.”

Trump, Pence and their top health-care aides have focused heavily during the past few days on the capacity to provide tests after a crescendo of criticism by Republican and Democratic lawmakers, as well as frustrated Americans.

The Centers for Disease Control and Prevention was slow at first to make and distribute an effective test as the virus began penetrating U.S. communities. While officials have emphasized that not everyone needs to get tested — and they appealed Sunday evening for people to reserve tests for those most at risk — public health experts have made clear that knowing where the virus has arrived is crucial to helping people avoid it.

On Friday, the Food and Drug Administration quickly approved a pair of manufacturers for coronavirus tests that can be run at far higher speeds than earlier options, and administration officials said the government needed to shift its focus from test supply to making it easier for Americans to get them.

The details described by the vice president were more modest than Trump portrayed during a news conference Friday in the White House’s Rose Garden, with flowering trees abloom.

“We have many, many locations behind us, by the way,” the president said, referring to chief executives of five big-box retailers and drugstores who agreed to donate parking lot space as testing sites. “We cover this country in large parts. . . . [W]e cover very, very strongly our country sourced in virtually every location.”

Moments later, the president invited to the lectern Deborah Birx, an ambassador and global AIDS specialist who has been brought into the vice president’s office to help coordinate the government’s coronavirus response. She held up a chart with a rough mock-up of how Americans could type in their symptoms and risk factors to a Google website and be told whether they warranted a test and where they could get one.

Together with newly approved higher-speed lab testing, “we want to bring this across the continent,” Birx said.

Following the news conference, it quickly became evident that the announcement, engineered by the office of Jared Kushner, Trump’s son-in-law and senior adviser, far exceeded the actual preparations.

Asked about the specific plans afterward, representatives of the four companies — Target, Walgreens, Walmart and CVS — said they had few details on how the tests would be administered or where or when they would begin.

And an hour after the president and his aides left the Rose Garden, a Google communications account tweeted a comment from Verily, the life sciences division of Google parent company Alphabet, that suggested the idea of building a broadly available website is preliminary.

“Verily is in the early stages of development, and planning to roll testing out in the Bay Area, with the hope of expanding more broadly over time.”

On Saturday, the Google communications account updated its tweet, saying: “Google is partnering with the U.S. government in developing a nationwide website that includes information about covid-19 symptoms, risk and testing information. . . . This in addition to . . . work being done by our sister company, Verily, to launch a pilot website that will enable individuals to do a risk assessment and be scheduled for testing at sites in the Bay Area.”

On Sunday, there were indications that the plans were still coalescing shortly before Pence’s announcement.

One person who works closely with the CDC on the coronavirus issue, speaking on the condition of anonymity to discuss a private conservation, said that key CDC lab officials had not been informed of any details of the plans 90 minutes before the White House was scheduled to make its announcement.

And with the White House briefing scant on details about where the drive-through tests would first become available, it was unclear which states FEMA and the Public Health Service have begun to coordinate.

“We are working closely with governors,” Pence said, noting that “more than 10 states” have started their own drive-through testing, including New York, Colorado, Delaware and Texas.

On Sunday, New York Gov. Andrew M. Cuomo’s (D) office said it was unaware of any official coordination between the state and the Trump administration to test residents in store parking lots. The governor did not mention any such steps during a lengthy afternoon news conference.

In the White House briefing room, Anthony S. Fauci, director of the National Institutes of Health’s National Institute of Allergy and Infectious Diseases and a veteran of decades of epidemics and pandemics, reminded Americans that “the worst is yet ahead of us” with the coronavirus.

“It’s how we respond to that challenge that will determine what the ultimate end point is going to be,” he said.

Darryl Fears and Jay Greene contributed to this report.