While insufficient testing limits the full picture, it’s clear the disease is spreading at different speeds in different places in the United States. California and Washington continue to see new cases and deaths, but so far they haven’t come in the spikes seen in parts of the East Coast. Social distancing efforts need to continue for several more weeks to be effective, experts say.

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The data give “great hope and understanding about what is possible,” said Deborah Birx, the White House coronavirus response coordinator, during a Tuesday briefing. “In New Orleans, and Detroit, and Chicago and Boston right now, [we’re] trying to make sure that each of those cities work more like California than the New York metro area.”

It has been 16 days since counties in the San Francisco Bay area told some 6 million residents to stay at home, and 13 days since the order extended to all of California. As of Tuesday, the number of confirmed infections per capita in densely populated New York City was 15 times that of the Bay Area. In New York City, a flood of coronavirus patients has overwhelmed local hospitals and 1,096 people have died. New York state ordered people to stay home 11 days ago.

Compared with the Boston area, which has a more-similar population density, California’s Bay Area has about a third of the of the cases, per capita. The state of Massachusetts ordered people to stay home 8 days ago.

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“Every aggressive action appears to have helped slow the spread,” San Francisco public health head Grant Colfax told The Washington Post. He warned on Tuesday about the virus potentially spreading in the 750-person long-term care Laguna Honda Hospital, but so far the city has seen a total of 397 confirmed cases and 6 deaths. The hardest-hit part of the Bay Area is Santa Clara County in Silicon Valley, which has seen 890 cases and 30 deaths as of Tuesday.

Aggressive social distancing efforts haven’t stopped the virus, public health experts say. But the goal was to slow the spread to keep it from overwhelming health-care resources so fewer people would require hospital beds and ventilators at the same time. California hospitals, which saw their number of covid-19 patients double over the last five days, have yet to buckle under the load.

“The ER is eerily quiet right now,” said Jahan Fahimi, an emergency care physician at UCSF Health in San Francisco. He credited early action by policymakers in the Bay Area, even as he braced for what’s next. “The surge is still coming. It’s not that we’ve averted anything,” he said.

Academic models of the coronavirus spread from the University of Washington’s Institute for Health Metrics and Evaluation, which Birx cited in her remarks, indicate California’s steps reduced the state’s total projected death toll from 6,100 to about 5,100. “We are seeing less deaths and flattening of the curve,” said Ali Mokdad, a senior faculty member at IHME. So far, California has reported 150 deaths.

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The IHME’s model, which projects some 94,000 deaths across the entire country, is updated daily with data from each state. It is designed to help hospitals and other decision-makers determine what they’ll need in terms of intensive-care beds and ventilators.

In Washington state, where the virus took off in a nursing home in February, officials first banned events of 250 people or more and closed schools in Seattle on March 11, ordered bars and restaurants to close on March 16, and then ordered the whole state to stay at home on March 23. Following those moves, IHME death projections for Washington fell from 2,000 originally to about 1,600. So far, 195 people have died in the state.

California Gov. Gavin Newsom (D) said Tuesday he wants to be “cautious” about drawing conclusions about the effectiveness of the state’s social distancing efforts but that he’s more confident its health care system can handle what’s coming.

“We have the time to prepare. That was the whole point of moving early on physical distancing,” Newsom said. “The only regret we will have is if we cut the parachute before we land.”

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Coronavirus models and the public policy game plan they’re informing are based on sophisticated guesswork. Since patients can take days or even weeks to show symptoms, case counts are a lagging indicator of the success of social distancing efforts.

Even then, not everyone who feels sick can get tested. “We’re really not sure of the size of the problem we’re facing,” said Warner Greene, director of the Gladstone Institute of Virology and Immunology in San Francisco.

California, in particular, hasn’t been able to test all the patients who think they’re sick. As of March 30, health officials report approximately 86,100 tests had been conducted in the most-populous state — yet 57,400 results were still pending.

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Jeanne Noble, director of covid-19 response for UCSF’s Parnassus emergency department, said testing has been a constant struggle and her hospital started to run out of critical nose swabs in mid-March. After reaching out to biotech firms and other private companies, they were promised 4,000 swabs from the strategic national reserve. Those were sent to the wrong place, causing another multiday delay.

“I think we are the only developed country who has lagged so poorly in our ability to test broadly,” Noble said. “We are still struggling with the very first step of figuring out who has the disease and who doesn’t, and that’s simply atrocious.”

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In lieu of widespread testing, academics and policymakers have been turning to other signals. Deaths offer a reliable view of the spread of the disease but can lag its growth by weeks and also increase when the health care system gets taxed. A more current index is hospitalizations, which California began to report recently.

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Newsom said his models included hospital use, as well as data about population movement gathered by tech companies.

The success of shelter-in-place orders in countries including China and South Korea already have given researchers an indication such measures work. But there are differences between countries and even within the United States that complicate direct comparisons.

Some lessons of social distancing from West Coast cities also could be geographically and economically unique. They’ve got a fraction of the population density of many East Coast areas. In contrast to New York City, many people drive cars even in the most-populous California and Washington cities rather than take trains and buses.

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“Los Angeles has the worst public transportation system in the country. Everyone drives. This has actually turned out to be protective for this kind of virus,” said Lee Riley, a professor of infectious disease at the University of California Berkeley.

The Seattle and San Francisco areas also are home to large tech companies including Facebook and Microsoft, both of which told their employees to work from home beginning in early March, a week before official orders. These communities, prosperous because of tech, tend to listen when tech leaders talk.

Nicholas Jewell, a professor of biostatistics at the University of California Berkeley, said sheltering in place one week earlier can make a huge difference in stopping the disease. He said it’s concerning that some states have yet to implement the policy. “States that are like ‘we need to keep our economy opened,’ that’s a mistake,” he said. “That’s the lesson we’ve learned over and over again in infectious diseases.”

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About 30 states — representing more than two-thirds of Americans — have issued stay-at-home orders with exceptions for “essential” workers. In some places, including Washington D.C., such orders just went in place this week.

“We’re trying to tell everybody that [shelter-at-home orders] work,” said Mokdad of IHME. He said his outreach included Florida’s surgeon general Scott A. Rivkees. “I told him that they are working everywhere else, and I gave him examples, and I said, ‘Please implement them.’” said Mokdad.

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Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, acknowledged during the White House news conference on Tuesday that social distancing is “inconvenient” for most people. “But this is going to be the answer to our problems.”

The glimmer of good news isn’t a sign cities and states can ease up on restrictions, say researchers. They emphasize it’s more important than ever that people stay home, and some regions are even adding to shelter-in-place orders. On Tuesday, the Bay Area extended its to the beginning of May.

If shelter-in-place orders are withdrawn, the virus can rebound, said San Francisco’s Colfax.

Shelter-at-home orders also won’t work on their own. As soon as an order ends, whether it’s in the start of May or later, the virus could circulate again and knock some areas back to where they started.

Weaning a city or state off lockdown requires something no area in the United States seems to have yet: an exhaustive number of tests.

“Once this first wave is over, we’ll have to get some kind of mass testing machine in order to get a situation where we don’t need [social distancing],” said University of Washington’s Christopher Murray, director of IHME. “We’ll have to do mass testing and contact tracing and some quarantines.”

Elizabeth Dwoskin and Joel Achenbach contributed to this report.

Graphics by Adrián Blanco.

Covid-19 cases come from the data reported by the different counties. Not all counties have been doing equal amounts of testing, which could skew views of confirmed case counts. Population data to calculate the ratio per 100,000 people comes from the census.

Santa Clara, San Francisco, Marin, Alameda, Berkeley, Contra Costa and San Mateo counties have been used to calculate the number of cases in the San Francisco Bay area. King, Pierce and Snohomish counties have been used to calculate the number of cases in the Seattle area. Middlesex, Suffolk, Norfolk and Essex counties have been used to calculate the Boston area numbers. Los Angeles data is based on the number of cases reported by Los Angeles County. Miami-Dade County and Broward County have been added to represent the number of cases in the Miami area.