As the criteria for who can get tested for the new coronavirus loosens, cities around the country are bracing for community spread in their regions — cases that are transmitted to people with no history of travel or traceable contact with a known positive case.

But in New Orleans, that day is already here. It may have arrived weeks ago, when safety at Mardi Gras parades was of greater concern than a virus many thought was still a hemisphere away.

Infectious disease expert Anthony Fauci, a member of the federal Coronavirus Task Force, named four states earlier this week where community spread of coronavirus makes the risk of infection and death higher than areas where cases can be traced to travel or other positive cases.

“You know the places: Washington state, California, New York and Florida,” Fauci said.

By midweek, Gov. John Bel Edwards had added Louisiana to that list, saying “we are fairly certain now that there is community spread in the Orleans area.” And by Friday afternoon, New Orleans Mayor LaToya Cantrell described “significant community spread” as the case count across the state jumped to 36, nearly all of them in the city and its suburbs.

Orleans Parish currently has more than half as many known cases as Los Angeles County, with about 4% of the population.

The disease may have been brewing for weeks in the city, a destination for travelers particularly during Carnival season, which ended two weeks ago. With its mix of thousands of tourists, a warm and humid climate and festivals that pack strangers into small spaces, New Orleans may be a prime breeding ground for the spread of a disease like COVID-19, according to experts.

Unlike in other places, officials said Louisiana’s first known case — in Jefferson Parish, announced Monday — was not related to travel or linked to a close contact diagnosed with COVID-19. That is cause for concern.

“It probably arrived around Mardi Gras,” said Mac Hyman, a Tulane University math professor who specializes in modeling diseases. “We have so many people around the world standing shoulder to shoulder for hours and hours.”

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Viruses also tend to live longer in humid environments, where a bit of water can extend their lifespan, Hyman said.

During outbreaks of disease, tracing the path of illness is crucial to identifying vectors — people carrying the disease — and limiting their activity. It can mean the difference between infecting just a few others and a few hundred others, said Hyman. But the time to identify and track cases in New Orleans may have been weeks ago, given that community spread is already happening.

“If the person infected is a bus driver, they’ll spread it all over town,” he said. “But if they live alone and never go out, they probably won’t spread it.”

The confirmed community spread early on in the fight against coronavirus in New Orleans makes it unlike most other places across the U.S., where the first known cases were traced to travel or someone known to be infected.

Texas, for example, had at least 50 cases as of Friday afternoon, but had only identified two as community spread. New York and Washington state, both considered hotspots of the virus, both first confirmed travel-related cases of coronavirus before identifying community spread.

The effort to track and stop the spread of a disease like coronavirus becomes more difficult once a community, like New Orleans, has cases that cannot be traced to travel or other cases, according to epidemiologist Susan Hassig, a professor at Tulane University.

“We are demonstrating you don’t need to have an external connection anymore for it to be present in a community,” said Hassig. “We are, like Italy and many other places, the new face of coronavirus.”

In Louisiana, 33% of tests taken as of Friday afternoon had come back presumptively positive. Samuel Scarpino, who runs the Emergent Epidemics Lab at Northeastern University in Boston, said that’s a sign of severe undertesting and under-reporting. In Washington state, by comparison, where testing is much more widespread, around 10% of tests have been positive.

“If you see 30% of the tests are positive, that’s just more indication that we’re off by many multiples of how many tests we need to be running,” he said.

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Scarpino, an assistant professor of network science, said the estimates he’s seeing of counts for coronavirus are least off by a factor of 10. He — and others — called for drastic action nationwide to help turn the tide of the disease.

Louisiana moved to close all K-12 schools, most universities and directed some state employees to work remotely. The state on Friday also banned gatherings of more than 250 people.

“It’s certainly too late to prevent widespread transmission; we know that’s happening,” Scarpino said. “It’s not too late to take decisive action now that will lower mortality rates and lower the chance we need to take extreme measures in the future.”

Numerous people in the New Orleans area who’d developed symptoms of coronavirus complained about their inability to get tested in emails and calls with reporters. Others voiced alarm that they might have unwittingly spread coronavirus to friends, relatives and coworkers after assuming lingering late February illnesses were regular cases of the post-Mardi Gras crud.

A lack of testing makes it difficult if not impossible to estimate the scope of the epidemic, experts said.

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“When you’re talking about case count, it really is the tip of the iceberg,” said Rebecca Christofferson, a professor at LSU School of Veterinary Medicine who specializes in transmission biology and emerging viruses. “We’re not in a position to capture silent transmission.”

Christofferson warned that closely following the number of known cases wouldn’t provide an up-to-date picture of the public health battle against the virus. The number of known infections will almost certainly continue to spike in the coming days as those already infected begin to show symptoms and as testing becomes more extensive — and dropping numbers of new confirmed cases won’t necessarily mean the fight is over.

The best data on the COVID-19 epidemic is coming from South Korea, Christofferson said, where extensive testing for the virus found contagious infections in people with few — if any — symptoms. The U.S. hasn’t rolled out that kind of large-scale testing, Christofferson noted, instead using its few testing kits to screen people who are already seriously ill.

That will change as cases spike and commercial testing becomes available.

“The attention that was given to travel restrictions and aimed at limiting the import of the virus now needs to be measured with limiting interactions in the community,” said Dr. Joe Kanter, an assistant state officer for the Louisiana Department of Health. “Contact tracing remains important, but as case counts grow, you will never be able to diagnose 100% of people.”

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Tad Dallas, an assistant professor at LSU’s Department of Biological Sciences, said there are “fundamental limits” to predicting the extent of an epidemic’s spread or estimating the size of the outbreak in Louisiana based on the state’s limited testing.

Forecasting with so many unknowns could offer a “dangerous” sense of certainty to the public, he said.

Data from early on in an epidemic, Dallas said, doesn’t always give a true picture of the scope of infection. Severely ill patients will be reported, while others with mild symptoms might go unnoticed, potentially skewing the picture both of the outbreak’s scope and its lethality.

Data from China suggest at least 80% of cases result in mild symptoms. Louisianans with mild symptoms, with few exceptions, aren’t being tested.

“It is likely that the novel coronavirus (COVID-19) will spread throughout the state of Louisiana,” Dallas said by email Friday, but added that public health measures like social distancing could have “a fairly rapid impact” on slowing the spread of the disease and protecting particularly vulnerable residents.

Both Dallas and Christofferson also warned that researchers still have much to learn about the novel coronavirus, which was identified just months ago.

“This particular group of viruses, coronaviruses, are difficult to forecast because we haven't seen that many of them,” Dallas said. “There are seven known to infect humans, which gives scientists a limited amount of knowledge to work from.”

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Edward Trapido, an epidemiologist with LSU Health Science Center’s School of Public Health in New Orleans, agreed the number of cases will likely spike once testing becomes more widespread, saying “we could easily see thousands.” But Trapido warned people not to panic.

Social distancing and other drastic action is important, he said, to protect the elderly, and others who lack the ability to fight off the disease as easily as the younger and healthier people who may be unknowingly causing community spread.

“What we’re seeing are the acute cases — the people who have gross symptoms and who end up seeking emergency healthcare,” Trapido said. “We know the tip of the iceberg, but without testing, we’re not going to know what the whole iceberg looks like.”

It will take time before people know whether the actions implemented throughout the week — particularly Friday’s closures — are effective at slowing the spread of coronavirus. Cases are going to rise before they plateau, the experts said.

“We’re going to see cases rising as test rates go up as we discover what the true effect is,” Scarpino said. “It’s going to be a little bit of time before we’re really in the position to look back and see what’s working, what’s not working and who ramped up fast enough. That’s why we’re too late in some regards, but we’re not too late to have an effect in the next two weeks, three weeks.”

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Christofferson said there’s “absolutely still a chance to flatten the curve” to avoid a potentially catastrophic surge of new cases that could overwhelm hospitals. She said she’d rather people overreact than underreact because it will help to stop the spread of transmission.

“I don’t know that we’re Italy, I don’t know that we’re South Korea. I think we might be somewhere in the middle,” Christofferson added. “I don’t know that we’re down a path of no return. We’re not quite that dire yet."