Researchers have found more evidence that coronavirus was circulating at low levels across the United States as early as December 2019 -- weeks before the first officially reported cases.

Frozen blood samples indicate people in five states -- Illinois, Wisconsin, Pennsylvania, Mississippi and Massachusetts -- were infected with coronavirus days or weeks before any cases were officially reported in those states.

https://edition.cnn.com/2021/06/15/health/coronavirus-all-of-us-blood-survey/index.html

Volunteers taking part in the National Institutes of Health's All of Us study, an ongoing effort to gather health information on 1 million people, donated blood as part of the study. Tests of 24,000 samples taken in early 2020 showed antibodies to coronavirus in the blood of at least nine people, the All of Us researchers reported in the journal Clinical Infectious Diseases.

"These included individuals with specimens collected January 7 from Illinois, January 8 from Massachusetts, February 3 from Wisconsin, February 15 from Pennsylvania, and March 6 in Mississippi," they wrote.
The first previously recognized case of Covid-19 in Illinois was reported on January 24 in a woman who had just returned from Wuhan, China, the researchers said.
The first confirmed case in Massachusetts was not until February 1. In Wisconsin, the first confirmed case had been February 5, in Pennsylvania the first reported case was March 6 and in Mississippi it was March 11.

Since it takes about two weeks to develop antibodies after infection, the findings indicate some of the volunteers were infected in December, the researchers said.
"This study contributes to the evidence of low-level circulation of SARS-CoV-2 in many states at the start of the US epidemic," the researchers wrote.

"Among the first 12 known cases of SARS-CoV-2 infection in the United States, the earliest recognized symptoms onset date was January 14, 2020, and all 12 cases had recently traveled to mainland China or were close contacts of recent travelers. Domestic testing for SARS-CoV-2 began in mid-January 2020," they added.
At the time. the federal government only recommended testing people with symptoms who had a history of travel, or direct contact with a traveler.
These findings suggest that policy missed cases, the researchers said.
"I think what this study shows is that there are a lot of pieces still that we have to uncover and put together from all the days of this epidemic in the United States," Dr. Keri Althoff, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health who worked on the study, told CNN.