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Somerville and L.A. are offering coronavirus testing for everyone. But what does that mean, and why isn’t Boston doing it?

A nurse at the Cambridge Alliance Hospital in Somerville administered coronavirus tests to patients on Tuesday.
A nurse at the Cambridge Alliance Hospital in Somerville administered coronavirus tests to patients on Tuesday.David L. Ryan/Globe Staff

When the mayor of Los Angeles announced this week that the city would be making COVID-19 testing available to all its residents, regardless of whether they were symptomatic, the announcement left some in Boston scratching their heads. Why could a sprawling city of roughly 4 million people offer universal testing, while Boston cannot?

As scientists have learned more about asymptomatic carriers who spread the coronavirus even when they don’t know they have it, widespread testing and subsequent isolation of people who test positive has been touted as essential to containing and ending this crisis. But most cities in Massachusetts and around the country allow testing only for people who show symptoms. This week, Somerville became the first city in the state to promise free testing with an appointment to all its residents.

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“As we are asking how to open up, I can’t stress three things enough: testing, testing, and more testing,’’ Mayor Joseph Curtatone of Somerville told the Globe.

The answer to the puzzle of why Somerville and Los Angeles could promise testing for all comers lies in differences in the size of Boston’s outbreak compared to the ones in those cities, as well as the fine-print of so-called universal testing.

Massachusetts, like other states across the country, has a finite number of tests and resources, including the lab supplies like cotton swabs and chemical reagents needed to administer them. Mayor Martin J. Walsh of Boston has frequently stressed the need for more tests.

“Ultimately, we’re going to need a strong federal commitment to get all the supplies we need to expand testing to all of Boston’s residents,” Walsh said in a statement. Instead of aiming for universal testing, the city said it has prioritized increasing testing capacity in some of the neighborhoods, like Hyde Park, Mattapan, and Dorchester, that have been hardest hit by the pandemic. Currently, Boston has 15 free testing sites; all require pre-screening and an appointment.

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The same is true in most of Massachusetts. In addition to its site in Somerville, Cambridge Health Alliance opened a testing site in Malden this week where residents from that city and Everett could get tested, regardless of symptoms. But even in Chelsea, the city that has the highest rate of coronavirus infections in the state, free testing is available only to people who have symptoms.

“Universal testing is certainly a plan that I think anybody in public health would say is great if you can do that," said Laura White, an associate professor of biostatistics at Boston University. But, she said, “it’s very challenging — making sure you have enough supplies, that you have safe places to test people, that you have the lab capacity to do those tests.”

Because states have a limited number of tests, those facing a more limited outbreak have more tests to spread around to people who may not be symptomatic, said Dr. Ashish Jha, director of the Harvard Global Health Institute. That’s part of the story when comparing Los Angeles to Boston. As of Friday, Los Angeles County had reported 231 cases and 11 deaths per 100,000 people, according to data collected by The New York Times. Suffolk County, in contrast, has experienced 1,628 cases and 66 deaths per 100,000 people, a much higher rate. Somerville, too, has had a lower disease outbreak than Boston, with about 748 cases per 100,000, as opposed to 1,335 in the city of Boston.

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“When you don’t have a big outbreak, it’s much, much easier to do universal testing, because you’re not using up all of your tests on people who are infected,” said Jha. In Boston, for example, "There’s so many people showing up at emergency rooms and calling up doctor’s offices with symptoms; those people are using up all the tests.”

Another factor to consider is what exactly “universal testing” means. The reality is that both Los Angeles and Somerville don’t have enough tests for all of their residents to get tested tomorrow.

Curtatone said the state sent materials to test 3,500 people after Somerville and Cambridge Health Alliance officials put forward a multilayered plan for testing and for connecting people with city services when necessary.

“We put in a pretty robust and integrated plan on how we are going to expand testing,” Curtatone said.

But the city has around 80,000 residents. Curtatone said interest in getting tested far outstrips the supply of tests, generating around 900 phone calls a day from people asking for an appointment. So it may be that everybody who wants a test in Somerville won’t get one, at least not soon, after all.

The city of Los Angeles had 16,000 booked appointments on the first day of its new universal policy.

“It’s not going to be 4 million people all rushing to the testing centers overnight,” said Mayor Eric Garcetti during a briefing announcing the city’s new testing policy. “We’re noticing, for instance, we’ve offered all of our firefighters testing. Not all of them are taking us up on it.”

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Los Angeles city officials are not expecting that demand for tests will far outstrip the supply, although appointments for asymptomatic people were fully booked on both Thursday and Friday mornings. That may be because the city is still prioritizing appointments for people with symptoms, as well as front-line workers. Appointments that are not claimed by those groups are allocated throughout the day to people who want to be tested but don’t have symptoms or other risk factors, a spokeswoman for the city said.

Garcetti explained that he made the decision to expand testing in Los Angeles because the city had so far not reached its testing capacity.

“We have been opening up each night to more and more people, and still at the end of the day we have some tests that are left over. So we had the confidence that we could move forward,” Garcetti said in a briefing. Previously the city had been ordering 3,500 test kits a day, and only 50 percent of appointments were filled daily. Now the city is ordering 10,000 tests a day.

The model of “universal testing" that both L.A. and Somerville have rolled out takes for granted that not everyone who is eligible will choose to get tested.

“What L.A. is doing is outpacing the demand for testing,” said Jha. “And Boston is not able to outpace the demand.”

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Los Angeles County has 34 testing sites, and the city has partnered with the nonprofit Community Organized Relief Effort, founded by the actor Sean Penn, to run some of them. While the city pays for the tests and the infrastructure of its sites, CORE pays for staff and personal protective equipment at the sites where it works.

A third-party vendor Los Angeles is using notifies people if they test positive and lets them notify others of their status anonymously.

“To be most effective, you would want to couple [universal testing] with contact tracing,” White said.

Experts agree that widespread testing is important to rein in the virus, although some said the goal did not need to be universal testing, but instead a low threshold for testing and targeted testing of particularly high-risk groups, including nursing home workers and residents, people who work in or enter the health care system, and front-line workers who interact with the public. Coupled with contact tracing, even slightly more expansive testing could make a big difference.

“At the heart of managing a pandemic, you want to keep infected people away from susceptible people,” said Dr. Jha. “When you can’t do that because you don’t have testing, you have to shut the whole economy down.”

This story has been updated to clarify how Los Angeles is using a third-party vendor to notify residents of possible contact with coronavirus patients.

CORRECTION: This story has been updated to clarify that Los Angeles city partnered with CORE to run only some of its testing sites.

John Ellement of the Globe staff contributed to this report.


Zoe Greenberg can be reached at zoe.greenberg@globe.com. Follow her on Twitter @zoegberg.