A small survey in early March of Santa Clara County residents who had respiratory symptoms but did not have the flu found that 11% of them tested positive for the coronavirus — striking findings that helped trigger an aggressive public health response and eventually the Bay Area shelter-in-place orders, according to a report released Friday.

The study, published by the Centers for Disease Control and Prevention, was among the first in the United States to use community surveillance — which involves testing large groups of individuals — to determine how widely the virus was spreading.

The results suggest that early, large-scale community surveillance across the country would have identified outbreaks early on, before they could spread beyond control, infectious disease experts said.

“Look at how easy this was to do. Why wasn’t everybody doing it? We could have jumped on this so much faster,” said John Swartzberg, a UC Berkeley infectious disease expert. “It didn’t break my heart to read this, but it put a little dent in it.”

In early March, the United States had about 200 cases. The country now has more than 250,000 cases, including 6,800 deaths — the largest outbreak in the world.

The CDC had discussed starting community surveillance as early as February but abandoned the effort due to a lack of testing supplies. The plan had been to tap into existing flu surveillance networks and do large-scale testing for the coronavirus. The surveillance was to take place in five cities, including San Francisco. But flaws in the tests produced by the CDC prevented that from happening.

That kind of surveillance could have given an early heads up to local public health officials that the virus was beginning to spread in their communities, and spurred them to take quick action to prevent outbreaks before they grew beyond their control, infectious disease experts said.

“This was something we here in Santa Clara County were very interested in, even a month before a study was able to be done,” said Dr. George Han, deputy health officer in Santa Clara County and senior author of the study. “There were limitations in getting something like this started. Ideally, we would have liked to have it earlier. And I think we weren’t alone. I think folks in health departments around the country would have liked it.”

The surveillance study was small due to the lack of testing supplies, and took place over a few days, from March 5 to March 14.

Santa Clara County issued its first social distancing guidelines — recommending that gatherings of more than 1,000 people be canceled — on March 9, while the study was still under way. The advisories grew increasingly more demanding over the following week.

The study was completed on March 14. By then, the number of cases reported in Santa Clara County had increased nearly fivefold since March 5 — from 20 to 91. The day after the study ended, Santa Clara County health officer Sara Cody called a meeting with other Bay Area public health officials and recommended a regional shelter-in-place order.

Six Bay Area counties announced the order on March 16, and it went into effect on March 17, making the Bay Area the first place in the country to begin sheltering in place. Gov. Gavin Newsom issued a statewide stay-home order a few days after that, and now more than half of the country is under some form of shelter-in-place directive.

But in some places the stay-home orders may have come too late. New York state residents began sheltering in place just a few days after Californians, but by then their outbreak was already out of control and now hospitals are overwhelmed with patients.

Community surveillance could have alerted public health officials to the virus’ arrival early enough to change the course of the outbreak, Swartzberg said.

“If we had been doing this nationally a month earlier, look at how informed we would have been,” he said. “People would have been extolling the virtues of public health interventions. It’s such a shame we weren’t able to.

“The books that are going to be written about this will point that out,” Swartzberg said.

Santa Clara County identified its first likely community-acquired case — meaning that the source of the individual’s infection could not be determined — on Feb. 27. More community cases were reported over the next few days, which prompted local, state and federal public health officials to start a study to help define how widespread the virus was.

Overall, 226 patients were included in the study, all of them Santa Clara County residents who had reported respiratory symptoms at four urgent care centers. Of those patients, 23% tested positive for the flu. A representative sample of the remaining patients were then tested for the coronavirus.

Of the 79 patients tested for the coronavirus, 11% of them — six women and three men — were positive. The study authors used that number to estimate that 8% of all patients with respiratory symptoms who were treated at the four urgent care centers were infected with the coronavirus. at that time.

“We probably should spend time congratulating Santa Clara County for initiating the testing and confirming community spread and implementing early interventions,” said Robert Siegel, a Stanford infectious disease expert. “That’s kind of the take-home message.”

State and local public health authorities are talking about starting new surveillance studies similar to the one in Santa Clara County, which would help officials understand whether sheltering in place is working. But testing shortfalls are still a problem, Han said.

“We need more testing, and there’s only one way to do that, and that’s to ramp up national production,” he said. “There needs to be a national strategy to this. Otherwise, testing is not happening fast enough to keep up with the epidemic.”

Erin Allday is a San Francisco Chronicle staff writer. Email: eallday@sfchronicle.com Twitter: @ErinAllday