The United States has so far reported the highest number of COVID-19 infections, with more than 1.86 million confirmed cases and over 106,000 deaths. The degree at which the new novel coronavirus is spread hints that the transmission may have started much earlier than previously thought.
Now, the U.S. Centers for Disease Control and Prevention (CDC) has said that the COVID-19 transmission in the United States began in late January or early February, meaning the virus has been spreading for over a month before the first case was detected.
Globally, the virus has infected more than 6.3 million and killed over 377,500 people.
In the United States, the first non-travel-related COVID-19 cases were confirmed on February 26 and 28, suggesting that the community transmission started happening by late February.
The federal government had detected 14 travel-related coronavirus cases between January 21 and February 23. The first local transmission case was reported on February 26 in California, while the second case was reported on February 28 in Washington.
The U.S. announced a ban on travelers from China on January 31 but it was implemented from February 3. Before that, the travelers were screened for symptoms such as fever and cough.
Investigators analyzed four lines of evidence – virus surveillance, syndromic surveillance, retrospectively identified cases, and virus genome analysis – to understand when and where the community transmission started.
The team found that the genetic analysis of earlier cases suggests the coronavirus was imported from China, which started spreading in the United States from January 18 to February 9, followed by many importations from Europe.
The investigators said there were several importations of the coronavirus from Europe to California and the northeastern United States in late February and early March.
They wrote, “Information from these diverse data sources suggests that limited community transmission of SARS-CoV-2 in the United States occurred between the latter half of January and the beginning of February, following the importation of SARS-CoV-2 from China.”
There is no indication the virus was introduced in the United States in November or December, according to the CDC.
“Steps are underway throughout the U.S. public health system to improve indicators of SARS-CoV-2 activity, including expanding syndromic surveillance among emergency departments and increasing the availability of testing for SARS-CoV-2,” the CDC researchers wrote. “Given the probability that most of the U.S. population is still susceptible, sustained efforts to slow the spread of the virus are crucial, including effective contact tracing and nonpharmaceutical interventions, such as physical distancing and source control (i.e., wearing cloth face coverings),” they added.
