Mardi Gras 2020 generated up to 50,000 cases of coronavirus, probably from a single source, says study | Coronavirus

Public health officials have widely accepted that last year’s Mardi Gras helped make New Orleans one of the first coronavirus concentration points in the United States, even though the lack of tests made it difficult to be sure.

But a new study that sought to identify how the virus spread throughout the city found that the Carnival 2020 revelry was responsible for tens of thousands of cases of coronavirus, after a single person probably brought it to New Orleans in the weeks before Mardi Gras .

Researchers at the Scripps Research Institute, Tulane University, LSU Health Shreveport and several other institutions said in a pre-publication report released on Monday that the coronavirus probably arrived in New Orleans about two weeks before Fat Tuesday. , probably from someone traveling from Texas.

They found that almost 800 people were probably infected when the crowds died on Ash Wednesday, and that these new infections continued to spread the majority of the 50,000 confirmed cases in Louisiana’s first wave in the following months.

“The entire outbreak in Louisiana is almost certainly fueled by what happened at Mardi Gras,” said Mark Zeller, author of the study and a researcher at Scripps. “It is basically the snowball. Once it’s going, it just goes. It is very difficult to stop. “

The newspaper, which has not yet gone through peer review, used death data to retroactively model how many cases the city would have. He employed genome sequencing to track cases to a single source and cell phone tracking data to help simulate the spread.

The findings, drawn from previous studies and reports focused on spreading in New Orleans, added further evidence that Mardi Gras was an over-spread event that changed the shape of the pandemic, both in Louisiana and in the southern United States.

And it suggests that the carnival season could hardly have been worse, with the festivities gaining momentum just before the closure was recommended by federal officials. It was also a period when providers had little experience in treating patients with coronavirus and a limited ability to test and monitor exposure.

“In March, I think New Orleans had the biggest increase in patients worldwide,” said Zeller. “It is a very unhappy time. If Mardi Gras had happened two, three weeks earlier, it might not have worked in so many cases. “

The virus spread through New Orleans differently than other cities with early spikes, such as Seattle and New York. In these places, several people probably brought slightly different strains in a short period of time, sowing mini-outbreaks that exploded quickly.

But based on an examination of the study of the genetic variety of the virus present in New Orleans, Mardi Gras essentially created conditions similar to that of a cruise ship, where a single variety of the virus spreads rapidly from a single source.

“It’s the same kind of situation,” said Robert Garry, a virologist at Tulane University and a leading author of the article. “On a cruise ship, people are huddled in smaller spaces, people are in these confined areas,” said Garry. “Almost like a Mardi Gras krewe, right? People spend a lot of time together, at dances, in these closed spaces. “

Garry said that in other cities, international travelers brought bloodlines from different places. But while scientists mapped the spread of the virus based on the genetic similarities between the cases, they found that the same was not true in Louisiana.

“People were concerned that a lot of travelers were coming from Asia and Europe with different strains, it didn’t,” said Garry. “He tells us about how this virus spreads – when people approach, talking, shouting and having fun, drinking, that’s where it spreads.”

The fact that only one major strain was transmitted was partly lucky and partly time, according to Jeremy Kamil, professor of microbiology and immunology at LSU Health Shreveport who helped coordinate the sequencing of coronavirus test samples. The unique pattern of a single Louisiana variety helped to illuminate Mardi Gras’ reach as an overspread.

“It’s like a spark,” said Kamil. “In Louisiana, by chance, there was more than one performance, but only one caught fire.”

Although larger areas, like Houston, had more strains of the virus, Mardi Gras seemed to trigger a chain of transmission that shaped the path of the pandemic that hit New Orleans, making it a city with the highest number of deaths per capita at times.

In a way, it was fortunate that only one lineage settled. But it happened at the most opportune time for a virus – when people huddled in crowds, sharing drinks and food, shouting and singing without thinking about a respiratory virus.

“It was a sweet spot in the pandemic, as it was a very early stage,” said Kamil.

Like humans, viruses transmit certain characteristics to each other. When scientists analyze all of these differences, an image begins to emerge, just like ancestors in a family tree.

“When you do that with New Orleans strains, they appear on a branch of the tree,” said Garry, “which means they were related.”

The article compared 235 sequences from the Louisiana genome with sequences from other parts of the country and the world. More than 30,000 pieces of each virus were analyzed and compared so that scientists could connect the dots between transmissions.

The document also dismisses the idea that Mardi Gras has sown infections in the United States. The strain of the virus in New Orleans has caused infections across the south – to other parts of Louisiana, Mississippi, Texas and Alabama – but restrictions on staying at home and limiting air travel seem to have prevented it from spreading to other regions.

While the city expects Mardi Gras that will look very different this year with closed bars, limited pedestrian traffic and banned canopies, the study’s authors hope their findings will serve as a warning about the effects of the collection as the virus circulates.

With new, more transmissible variants in circulation, the transmission chain is even more difficult to break.

“If college students are stationed on Bourbon Street, many of these people will not be seriously ill,” said Kamil. “But if they go home and visit grandma, or spread to someone else who passes it on to someone else who works in a nursing home or is a prison employee, then suddenly, you are indirectly involved in a chain of events where now you somehow participated in the deaths of 90 people. “