US COVID-19 Spread Likely Started Earlier Than Thought

Megan Brooks

May 29, 2020

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New evidence suggests that sustained, community spread of SARS-CoV-2 in the United States likely occurred earlier than thought (before detection of the first two nontravel-related cases) and likely started with a single infected person arriving from China.

The first two nontravel-related cases of COVID-19 in the United States were confirmed on February 26 and 28, 2020, suggesting that community transmission was occurring by late February.

However, four separate lines of evidence ― syndromic surveillance, virus surveillance, phylogenetic analysis, and retrospectively identified cases ― now suggest that US community transmission likely began earlier, in the latter half of January or early February, after a single case was imported from China.

An analysis of viral RNA sequences from the early cases suggested that a single lineage of virus imported directly or indirectly from China began circulating here between January 18 and February 9, according to the US Centers for Disease Control and Prevention (CDC) COVID-19 Response Team.

"This importation initiated a lineage, the Washington State clade, which subsequently spread throughout the Seattle metropolitan area and possibly elsewhere," they say.

This was likely followed by multiple imported cases from Europe in February and March. However, before February 28, overall disease incidence was too low to be detected through emergency department syndromic surveillance data, they note.

The CDC COVID-19 Response Team and colleagues report their findings in an article published online May 29 in Morbidity and Mortality Weekly Report.

Early Cryptic Circulation

The research team notes that the precise dates the virus landed in the United States and the identities of the persons who carried the virus remain unknown.

One possibility is a man from Washington State who became ill on January 19 after returning from Wuhan, China, on January 15. He represents the first reported US case. The genomic sequence of the virus isolated from this man is consistent with his being the possible source of the Washington State clade, the CDC researchers say.

In addition, the occurrence of three cases, one in a California resident who died on February 6, a second in another resident of the same county who died February 17, and a third in an unidentified passenger or crew member aboard a Pacific cruise ship that left San Francisco on February 11, "confirms cryptic circulation of the virus by early February," they report.

Amesh Adalja, MD, infectious disease physician and senior scholar at the Johns Hopkins Center for Health Security, Baltimore, Maryland, is not surprised by the new data.

"It's what most of us in the field have thought because this was an efficiently spreading respiratory virus that was present in China at least since mid-November. We also now know that there were cases in Europe as early as Christmastime," he told Medscape Medical News.

If the United States had taken action in January, "we would have had a much stronger hold on the virus," Adalja said. The United States didn't declare a national emergency until March 13.

"We know that viruses like these are not containable, and so we did suspect that there were cases that were going to [occur] all over the world by the time we even noticed them and that they were going to be mixed in with cold and flu season and may not have been detected," Adalja added.

And there never was going to be just one source, Adalja said. "That's why focusing on travel bans and screening and testing only on people who had been to China wasn't the correct strategy, because this virus had already escaped China even before we even identified it," he told Medscape Medical News.

The authors and Adalja have disclosed no relevant financial relationships.

Morb Mortal Wkly Rep. Published online May 29, 2020. Full text

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