Residents of six Bay Area counties, including Santa Clara and San Mateo, are being ordered to stay at home for all but "essential reasons" for the next three weeks, as the cases of COVID-19 continue to rise.

The order, which starts at 12:01 a.m. Tuesday, affects Santa Clara, San Mateo, San Francisco, Marin, Contra Costa and Alameda counties and the city of Berkeley and is designed to slow the spread of COVID-19, public health officials said.

The order limits activity, travel and business functions to only the most essential needs and defines essential activities as those necessary for the health and safety of individuals and their families. Residents will still be allowed to leave the house to pick up groceries or medication, but they are asked to stay at least 6 feet away from other people during these outings.

Essential businesses allowed to operate include health care operations; businesses that provide food, shelter, and social services, and other necessities of life for economically disadvantaged or otherwise needy individuals; fresh and non-perishable food retailers including convenience stores; pharmacies; child care facilities; gas stations; banks; laundry businesses and services necessary for maintaining the safety, sanitation and essential operation of a residence. In addition, health care, law and safety, and essential government functions will continue under the recommended action. The complete list of essential services and businesses is listed in the order.

San Jose Mayor Sam Liccardo said that no shortage of food is anticipated.

"No one need fear that they need to go buy everything on the (grocery store) aisle," Liccardo said.

However, for those who are vulnerable or at-risk, San Jose is coordinating with other cities to set up food-distribution programs throughout Santa Clara County that will commence in the coming days. This could also involve businesses, such as DoorDash, to help with food distribution to vulnerable seniors and people in their homes. Schools are setting up their own food-distribution networks, he said.

“We must move aggressively. The time for half measures is over. History will not forgive us for waiting an hour longer,” he said.

Most businesses are required to cease activities beyond minimum basic operations. Public and private gatherings are prohibited and all travel is prohibited except what is essential. Mass transit remains open, but the public should use it only for essential travel and are required to maintain social distancing as much as possible.

"While the goal is to limit groups congregating together in a way that could further spread the

virus, it is not a complete social shutdown. You can still complete your most essential outings or even engage in outdoor activity, so long as you avoid close contact," Dr. Matt Willis, Marin County’s public health officer, said.

"You can even take your dog for a walk. We are not expecting empty streets," he said during a press conference at 1 p.m. in Santa Clara County. "Social distancing does not mean disconnection from each other."

Dr. Sara Cody, Santa Clara County public health officer, said: "If I thought last Friday’s order was hard, this one is exponentially harder."

"Temporarily changing our routine is absolutely necessary to slow the spread of this pandemic," Cody said. "The health officers from the largest jurisdictions in the San Francisco Bay Area are united and we are taking this step together to offer the best protection to our respective communities."

Anyone who is ill with symptoms of the virus -- mainly fever, cough and shortness of breath -- needs to stay home, she added and to distance themselves from others in their household.

Homeless residents are exempt from the order, but local governments are encouraged to provide shelter for their homeless population, Cody said.

The guidance comes after substantial input from the U.S. Centers for Disease Control and Prevention (CDC) and best practices from other health officials around the world, the officials added.

Scientific evidence shows social distancing is one of the most effective approaches to slow the transmission of communicable disease. The shelter-at-home order follows new data of increasing local transmission of COVID-19, including 273 confirmed cases of COVID-19 with 4 deaths shared by the seven jurisdictions, as of 5 p.m. on March 15. The Bay Area’s collective confirmed cases are more than half of California’s case count, the officials said.

The number doesn’t account for rapidly increasing cases of community transmission, which are expected to grow significantly as testing for the virus expands. Testing capabilities for the virus are increasing though commercial laboratories and other sites, and as the number of people who are tested increases, the number of people who test positive for the infection is expected to rise dramatically, Cody said.

There should also be a greater percentage of people who test positive for the disease who are not hospitalized compared to those who are, she added.

The new, multiple-county legal order comes one day after Governor Gavin Newsom ordered older adults ages 65 and older to stay home. Persons most vulnerable to severe infection by the disease are ages 60 and older, those with chronic and underlying medical conditions and people experiencing homelessness.

Cody said the orders were crafted quickly with an eye to preserving essential social infrastructure. Santa Clara County is the epicenter of the outbreak, but because of the rapid increase in cases and number of hospitalizations, there became a need to act swiftly and regionally.

One of the reasons to slow the spread as quickly as possible is to protect health care workers from the virus so that the health care system won’t be overtaxed, she said.

The counties will post detailed FAQs on their websites to explain the order, Cody said. Liccardo said it will take time to work out details of enforcement of the order. It will be up to local officials to determine what constitutes "essential workers" in their community, a definition that will be much broader than police and fire and that is expected to include social workers, health care workers and utility employees.

Standing together - but six feet apart from each other - the health officers asked residents to unite as a community and to follow their advice.

"We are in a rough place," said Scott Morrow, San Mateo County health officer. "And we are going to have difficult times ahead of us.

"The measures we're putting in place are temporary but they will last longer than any of us want. This is the time to unite as a community, come to each other’s aid and dig really deep."

More information about COVID-19 is available on Alameda, Contra Costa, Marin, Santa Clara, San Francisco, San Mateo, or Berkeley COVID-19 websites.

A livestream of the announcement is archived at on the Facebook page of Santa Clara County Public Health Department.

This is a developing story. Check back here for details.

Find comprehensive coverage on the Midpeninsula's response to the new coronavirus by Palo Alto Online, the Mountain View Voice and Almanac here.