May 30, 2020 - Community transmission of the coronavirus in the U.S. probably began in late January or early February, according to a new CDC report released Friday.
Until late February, COVID-19 case numbers were too low to be detected by the CDC’s surveillance system that looks for flu-like illnesses based on emergency department records across the country.
“Enhanced syndromic and virus surveillance will be needed to monitor COVID-19 trends for the duration of the pandemic,” the CDC COVID-19 Response Team wrote in the report.
Between Jan. 21 and Feb. 23, the U.S. had 14 COVID-19 cases that were related to travel from China. The first non-travel case was confirmed on Feb. 26 in a California resident who became ill on Feb. 13, according to the report. Two days later, a second non-travel case was confirmed in Washington. That means that the virus was spreading in communities by late February, the team concluded, and the first cases came weeks before that.
The team looked at four lines of evidence — syndromic surveillance, virus surveillance, retrospectively identified cases and virus genome analysis — to understand when and where community transmission began.
Syndromic surveillance in counties that were affected early in the pandemic didn’t show an increase in emergency room visits for COVID-19 before Feb. 28, they wrote. Retrospective testing of 11,000 samples didn’t find positive samples before Feb. 20.
In Santa Clara County, California, two people had COVID-19 earlier than that, and they didn’t travel internationally. One woman became ill on Jan. 31 and died on Feb. 6. An unrelated man died at home between Feb. 13 and 17. Their tissues were tested postmortem and confirmed for COVID-19.
“Investigation of these cases is ongoing,” the CDC team wrote.
The team also analyzed the virus RNA from early cases and concluded that a single lineage of the virus was imported from China into Seattle and began circulating in the U.S. between Jan. 18 and Feb. 9, followed by several other lineages that were imported from Europe into California and the Northeast in February and March.
“Given the probability that most of the U.S. population is still susceptible, sustained efforts to slow the spread of the virus are crucial,” they wrote in the paper’s conclusion, including contact tracing, physical distancing and cloth face coverings.