Public officials have ordered increasingly stringent measures — closing schools, restricting travel, banning large gatherings — to slow the spread of the coronavirus.

Here’s why: They want to lessen the burden on hospitals, which experts say do not have enough staff, ICU beds or ventilators to withstand a surge of patients that an extreme pandemic would bring. On Saturday, the U.S. surgeon general urged hospitals to delay elective surgeries to preserve medical supplies and ease the burden on health care workers.

The vast majority of people diagnosed with COVID-19, about 80%, do not need medical care and will recover on their own. About 14% have to be hospitalized, and 5% end up in the ICU with severe respiratory problems, according to large studies on cases in China. Of those, many need a ventilator to help them breathe.

Infectious disease experts estimate the U.S. is about two to four weeks behind Italy, where hospitals are being flooded with patients with severe respiratory illness, and a shortage of beds and medical supplies is forcing doctors to have to choose which patients to treat.

“We’re hearing reports out of Italy of shortages of ventilators and ICU beds, that’s really concerning,” said Dr. David Eisenman of UCLA’s Fielding School of Public Health. “We’re hearing reports of health care teams, doctors and nurses, having to make choices about who gets a ventilator and who does not. ... That’s exactly what we worry about, that’s the exact scenario — if there’s a 50-year-old person who’s sick and needs an ICU bed and a ventilator, and a 75-year-old person who needs an ICU bed and a ventilator, and you have to make choices.”

It is unclear if the U.S. is heading down the same path. Social distancing and the cancellation of large events could help avert such a disaster by slowing transmission enough so that the influx of patients is spread over a longer period of time instead of all at once — a concept known as “flattening the curve.” Effective Saturday, San Francisco even banned “nonessential” hospital visits — including by spouses and clergy — in an effort to keep patients and medical workers safe, though each hospital can make exceptions.

The general efforts “will help,” said Dr. Andra Blomkalns, who chairs the Department of Emergency Medicine at Stanford. “We don’t honestly know if it will be enough.”

That’s in part because testing has been so delayed and limited in the U.S., public health officials haven’t been able to fully grasp yet just how widespread the virus is in the community — which makes it harder for hospitals to predict how dramatic of a surge to expect.

“Every day we get more information and try to guess,” Blomkalns said. “Right now is the time for overestimating what you need because we can’t accurately predict.”

Stanford is trying to triple its supply of ventilators from about 50 to 150 “ASAP,” Blomkalns said. Administrators are going outside their usual circle of suppliers and are having some luck, she said, because manufacturers are ramping up production.

A spokeswoman for GE Healthcare, a major supplier of hospital equipment, said the company is taking steps to increase manufacturing capacity for ventilators and other equipment needed to diagnose and treat COVID-19, such as ultrasound devices and monitors. Philips, another large supplier, said it is seeing “increased interest” in ventilators and is encouraging health care providers to “engage with us as early as possible to discuss their needs, so that we can plan for timely support and delivery,” a company spokesman said.

Stanford has about 60 ICU beds, and would be able to expand that pretty quickly by converting regular beds into ICU beds by adding ICU pumps and ventilators, she said.

There are about 1,550 ICU beds across all hospitals in the nine Bay Area counties, according to the California Department of Public Health. The department does not track how many ventilators there are at hospitals. Most of the regional hospital systems contacted by The Chronicle either declined to share how many ventilators they have or did not respond to questions about how many they have, whether they have done projections for how many they may need, if they are trying to obtain more, or how many they would be able to add quickly.

It is unclear how many ICU beds or ventilators would be enough to treat severely ill COVID-19 patients who may flood Bay Area hospitals in the coming weeks in need of intensive care.

“A surge in activity, the hospitals certainly in the Bay Area are capable of handling,” said Dr. John Swartzberg, an infectious disease expert at UC Berkeley. “A significant surge in activity of the virus, hospitals will handle, but it’ll be a scramble. A major surge in cases will overwhelm any health system in the world, including ours.”

It’s unclear how many of the 197 Bay Area cases have required treatment in the ICU because county health departments do not always specify what level of treatment each person receives. The first known case of community spread in the Bay Area, a Solano County resident who was admitted to a Vacaville hospital and later transferred to UC Davis Medical Center, was ill enough to be intubated and put on a ventilator.

The federal government has not released modeling projections for the virus, but if previous pandemics are any indication, the number of patients who may need ICU treatment in the U.S. could range from 200,000 to 2.9 million, according to a February report by the Johns Hopkins Center for Health Security.

The low end of the estimate, 200,000, is based on a moderate pandemic, the 1968 flu, which had a fatality rate of 0.5%. The high end of the estimate, 2.9 million, is based on a severe pandemic, the 1918 flu, which had a fatality rate of 2.5% in the United States.

It is not yet clear where COVID-19 falls in this continuum. Based on data collected so far, the fatality rate appears to be between 1% and 4%, though it may be lower since public health officials believe many cases have not been reported because people either did not show serious enough symptoms to report it or because they thought they had the flu or a cold. Mortality rates climb with age: People in their 60s face a nearly 4% chance of dying, and that rises to 8% for people in their 70s and 15% for people in their 80s, according to data on Chinese cases.

U.S. hospitals have 160,000 ventilators and an additional 9,000 in a strategic national stockpile managed by the CDC that can be requested and allocated during a pandemic, according to the Johns Hopkins Center for Health Security.

Clinics and hospitals that serve low-income populations could be particularly vulnerable if they see a sudden surge in patients with severe enough symptoms to need ventilators.

“Community hospitals don’t have a lot of ventilators,” Swartzberg said. “They have enough to take care of the patients they have on a regular basis and little bit of reserve. Just a few patients coming into any one hospital that needs a ventilator is going to exceed the ability of the hospital to care for them.”

Nearly 71% of community health centers say they are concerned about the ability of their facility to manage a surge in COVID-19 patients, according to a survey of more than 600 U.S. health centers conducted the first week of March by Direct Relief, a Santa Barbara nonprofit that distributes medical supplies during natural disasters and health crises.

“What we don’t know is, are we in the beginning of the end or the beginning of something terrible?” Swartzberg said. “We’re at the beginning of something. The question is which direction are we going to go? Either we take action, and try to make this outcome as good as possible, or we just give ourselves up to fate. It seems we ought to be taking action.”

Catherine Ho is a San Francisco Chronicle staff writer. Email: cho@sfchronicle.com Twitter: Cat_Ho